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Available in 38 languages" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-38" aria-hidden="true" ><span class="vector-icon mw-ui-icon-language-progressive mw-ui-icon-wikimedia-language-progressive"></span> <span class="vector-dropdown-label-text">38 languages</span> </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-ar mw-list-item"><a href="https://ar.wikipedia.org/wiki/%D8%AA%D9%88%D9%82%D9%8A%D8%AA_%D8%B0%D8%B1%D9%8A_%D8%B9%D8%A7%D9%84%D9%85%D9%8A" title="توقيت ذري عالمي – Arabic" lang="ar" hreflang="ar" data-title="توقيت ذري عالمي" data-language-autonym="العربية" data-language-local-name="Arabic" class="interlanguage-link-target"><span>العربية</span></a></li><li class="interlanguage-link interwiki-az mw-list-item"><a href="https://az.wikipedia.org/wiki/Atom_vaxt%C4%B1" title="Atom vaxtı – Azerbaijani" lang="az" hreflang="az" data-title="Atom vaxtı" data-language-autonym="Azərbaycanca" data-language-local-name="Azerbaijani" class="interlanguage-link-target"><span>Azərbaycanca</span></a></li><li class="interlanguage-link interwiki-be mw-list-item"><a href="https://be.wikipedia.org/wiki/%D0%9C%D1%96%D0%B6%D0%BD%D0%B0%D1%80%D0%BE%D0%B4%D0%BD%D1%8B_%D0%B0%D1%82%D0%B0%D0%BC%D0%BD%D1%8B_%D1%87%D0%B0%D1%81" title="Міжнародны атамны час – Belarusian" lang="be" hreflang="be" data-title="Міжнародны атамны час" data-language-autonym="Беларуская" data-language-local-name="Belarusian" class="interlanguage-link-target"><span>Беларуская</span></a></li><li class="interlanguage-link interwiki-be-x-old mw-list-item"><a href="https://be-tarask.wikipedia.org/wiki/%D0%9C%D1%96%D0%B6%D0%BD%D0%B0%D1%80%D0%BE%D0%B4%D0%BD%D1%8B_%D0%B0%D1%82%D0%B0%D0%BC%D0%BD%D1%8B_%D1%87%D0%B0%D1%81" title="Міжнародны атамны час – Belarusian (Taraškievica orthography)" lang="be-tarask" hreflang="be-tarask" data-title="Міжнародны атамны час" data-language-autonym="Беларуская (тарашкевіца)" data-language-local-name="Belarusian (Taraškievica orthography)" class="interlanguage-link-target"><span>Беларуская (тарашкевіца)</span></a></li><li class="interlanguage-link interwiki-bs mw-list-item"><a href="https://bs.wikipedia.org/wiki/Me%C4%91unarodno_atomsko_vrijeme" title="Međunarodno atomsko vrijeme – Bosnian" lang="bs" hreflang="bs" data-title="Međunarodno atomsko vrijeme" data-language-autonym="Bosanski" data-language-local-name="Bosnian" class="interlanguage-link-target"><span>Bosanski</span></a></li><li class="interlanguage-link interwiki-ca mw-list-item"><a href="https://ca.wikipedia.org/wiki/Temps_at%C3%B2mic_internacional" title="Temps atòmic internacional – Catalan" lang="ca" hreflang="ca" data-title="Temps atòmic internacional" data-language-autonym="Català" data-language-local-name="Catalan" class="interlanguage-link-target"><span>Català</span></a></li><li class="interlanguage-link interwiki-cs mw-list-item"><a href="https://cs.wikipedia.org/wiki/Mezin%C3%A1rodn%C3%AD_atomov%C3%BD_%C4%8Das" title="Mezinárodní atomový čas – Czech" lang="cs" hreflang="cs" data-title="Mezinárodní atomový čas" data-language-autonym="Čeština" data-language-local-name="Czech" class="interlanguage-link-target"><span>Čeština</span></a></li><li class="interlanguage-link interwiki-de mw-list-item"><a href="https://de.wikipedia.org/wiki/Internationale_Atomzeit" title="Internationale Atomzeit – German" lang="de" hreflang="de" data-title="Internationale Atomzeit" data-language-autonym="Deutsch" data-language-local-name="German" class="interlanguage-link-target"><span>Deutsch</span></a></li><li class="interlanguage-link interwiki-et mw-list-item"><a href="https://et.wikipedia.org/wiki/Rahvusvaheline_aatomiaeg" title="Rahvusvaheline aatomiaeg – Estonian" lang="et" hreflang="et" data-title="Rahvusvaheline aatomiaeg" data-language-autonym="Eesti" data-language-local-name="Estonian" class="interlanguage-link-target"><span>Eesti</span></a></li><li class="interlanguage-link interwiki-es mw-list-item"><a href="https://es.wikipedia.org/wiki/Tiempo_At%C3%B3mico_Internacional" title="Tiempo Atómico Internacional – Spanish" lang="es" hreflang="es" data-title="Tiempo Atómico Internacional" data-language-autonym="Español" data-language-local-name="Spanish" class="interlanguage-link-target"><span>Español</span></a></li><li class="interlanguage-link interwiki-eo mw-list-item"><a href="https://eo.wikipedia.org/wiki/Tempo_atoma_internacia" title="Tempo atoma internacia – Esperanto" lang="eo" hreflang="eo" data-title="Tempo atoma internacia" data-language-autonym="Esperanto" data-language-local-name="Esperanto" class="interlanguage-link-target"><span>Esperanto</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%D8%B2%D9%85%D8%A7%D9%86_%D8%A8%DB%8C%D9%86%E2%80%8C%D8%A7%D9%84%D9%85%D9%84%D9%84%DB%8C_%D8%A7%D8%AA%D9%85%DB%8C" title="زمان بین‌المللی اتمی – Persian" lang="fa" hreflang="fa" data-title="زمان بین‌المللی اتمی" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/Temps_atomique_international" title="Temps atomique international – French" lang="fr" hreflang="fr" data-title="Temps atomique international" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-ko mw-list-item"><a href="https://ko.wikipedia.org/wiki/%EA%B5%AD%EC%A0%9C%EC%9B%90%EC%9E%90%EC%8B%9C" title="국제원자시 – Korean" lang="ko" hreflang="ko" data-title="국제원자시" data-language-autonym="한국어" data-language-local-name="Korean" class="interlanguage-link-target"><span>한국어</span></a></li><li class="interlanguage-link interwiki-hi mw-list-item"><a href="https://hi.wikipedia.org/wiki/%E0%A4%85%E0%A4%A8%E0%A5%8D%E0%A4%A4%E0%A4%B0%E0%A4%B0%E0%A4%BE%E0%A4%B7%E0%A5%8D%E0%A4%9F%E0%A5%8D%E0%A4%B0%E0%A5%80%E0%A4%AF_%E0%A4%AA%E0%A4%B0%E0%A4%AE%E0%A4%BE%E0%A4%A3%E0%A5%8D%E0%A4%B5%E0%A4%BF%E0%A4%95_%E0%A4%95%E0%A4%BE%E0%A4%B2" title="अन्तरराष्ट्रीय परमाण्विक काल – Hindi" lang="hi" hreflang="hi" data-title="अन्तरराष्ट्रीय परमाण्विक काल" data-language-autonym="हिन्दी" data-language-local-name="Hindi" class="interlanguage-link-target"><span>हिन्दी</span></a></li><li class="interlanguage-link interwiki-hr mw-list-item"><a href="https://hr.wikipedia.org/wiki/Me%C4%91unarodno_atomsko_vrijeme" title="Međunarodno atomsko vrijeme – Croatian" lang="hr" hreflang="hr" data-title="Međunarodno atomsko vrijeme" data-language-autonym="Hrvatski" data-language-local-name="Croatian" class="interlanguage-link-target"><span>Hrvatski</span></a></li><li class="interlanguage-link interwiki-id mw-list-item"><a href="https://id.wikipedia.org/wiki/Waktu_Atom_Internasional" title="Waktu Atom Internasional – Indonesian" lang="id" hreflang="id" data-title="Waktu Atom Internasional" data-language-autonym="Bahasa Indonesia" data-language-local-name="Indonesian" class="interlanguage-link-target"><span>Bahasa Indonesia</span></a></li><li class="interlanguage-link interwiki-it mw-list-item"><a href="https://it.wikipedia.org/wiki/Tempo_atomico_internazionale" title="Tempo atomico internazionale – Italian" lang="it" hreflang="it" data-title="Tempo atomico internazionale" data-language-autonym="Italiano" data-language-local-name="Italian" class="interlanguage-link-target"><span>Italiano</span></a></li><li class="interlanguage-link interwiki-he mw-list-item"><a href="https://he.wikipedia.org/wiki/%D7%94%D7%96%D7%9E%D7%9F_%D7%94%D7%90%D7%98%D7%95%D7%9E%D7%99_%D7%94%D7%91%D7%99%D7%9F-%D7%9C%D7%90%D7%95%D7%9E%D7%99" title="הזמן האטומי הבין-לאומי – Hebrew" lang="he" hreflang="he" data-title="הזמן האטומי הבין-לאומי" data-language-autonym="עברית" data-language-local-name="Hebrew" class="interlanguage-link-target"><span>עברית</span></a></li><li class="interlanguage-link interwiki-la mw-list-item"><a href="https://la.wikipedia.org/wiki/Tempus_Atomicum_Inter_Gentes" title="Tempus Atomicum Inter Gentes – Latin" lang="la" hreflang="la" data-title="Tempus Atomicum Inter Gentes" data-language-autonym="Latina" data-language-local-name="Latin" class="interlanguage-link-target"><span>Latina</span></a></li><li class="interlanguage-link interwiki-lb mw-list-item"><a href="https://lb.wikipedia.org/wiki/International_Atomz%C3%A4it" title="International Atomzäit – Luxembourgish" lang="lb" hreflang="lb" data-title="International Atomzäit" data-language-autonym="Lëtzebuergesch" data-language-local-name="Luxembourgish" class="interlanguage-link-target"><span>Lëtzebuergesch</span></a></li><li class="interlanguage-link interwiki-lt mw-list-item"><a href="https://lt.wikipedia.org/wiki/Atominis_laikas" title="Atominis laikas – Lithuanian" lang="lt" hreflang="lt" data-title="Atominis laikas" data-language-autonym="Lietuvių" data-language-local-name="Lithuanian" class="interlanguage-link-target"><span>Lietuvių</span></a></li><li class="interlanguage-link interwiki-hu mw-list-item"><a href="https://hu.wikipedia.org/wiki/Nemzetk%C3%B6zi_atomid%C5%91" title="Nemzetközi atomidő – Hungarian" lang="hu" hreflang="hu" data-title="Nemzetközi atomidő" data-language-autonym="Magyar" data-language-local-name="Hungarian" class="interlanguage-link-target"><span>Magyar</span></a></li><li class="interlanguage-link interwiki-ms mw-list-item"><a href="https://ms.wikipedia.org/wiki/Waktu_Atom_Antarabangsa" title="Waktu Atom Antarabangsa – Malay" lang="ms" hreflang="ms" data-title="Waktu Atom Antarabangsa" data-language-autonym="Bahasa Melayu" data-language-local-name="Malay" class="interlanguage-link-target"><span>Bahasa Melayu</span></a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/%E5%9B%BD%E9%9A%9B%E5%8E%9F%E5%AD%90%E6%99%82" title="国際原子時 – Japanese" lang="ja" hreflang="ja" data-title="国際原子時" data-language-autonym="日本語" data-language-local-name="Japanese" class="interlanguage-link-target"><span>日本語</span></a></li><li class="interlanguage-link interwiki-nn mw-list-item"><a href="https://nn.wikipedia.org/wiki/Internasjonal_atomtid" title="Internasjonal atomtid – Norwegian Nynorsk" lang="nn" hreflang="nn" data-title="Internasjonal atomtid" data-language-autonym="Norsk nynorsk" data-language-local-name="Norwegian Nynorsk" class="interlanguage-link-target"><span>Norsk nynorsk</span></a></li><li class="interlanguage-link interwiki-pl mw-list-item"><a href="https://pl.wikipedia.org/wiki/Mi%C4%99dzynarodowy_czas_atomowy" title="Międzynarodowy czas atomowy – Polish" lang="pl" hreflang="pl" data-title="Międzynarodowy czas atomowy" data-language-autonym="Polski" data-language-local-name="Polish" class="interlanguage-link-target"><span>Polski</span></a></li><li class="interlanguage-link interwiki-pt mw-list-item"><a href="https://pt.wikipedia.org/wiki/Tempo_At%C3%B4mico_Internacional" title="Tempo Atômico Internacional – Portuguese" lang="pt" hreflang="pt" data-title="Tempo Atômico Internacional" data-language-autonym="Português" data-language-local-name="Portuguese" class="interlanguage-link-target"><span>Português</span></a></li><li class="interlanguage-link interwiki-ro mw-list-item"><a href="https://ro.wikipedia.org/wiki/Timpul_atomic_interna%C8%9Bional" title="Timpul atomic internațional – Romanian" lang="ro" hreflang="ro" data-title="Timpul atomic internațional" data-language-autonym="Română" data-language-local-name="Romanian" class="interlanguage-link-target"><span>Română</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/%D0%9C%D0%B5%D0%B6%D0%B4%D1%83%D0%BD%D0%B0%D1%80%D0%BE%D0%B4%D0%BD%D0%BE%D0%B5_%D0%B0%D1%82%D0%BE%D0%BC%D0%BD%D0%BE%D0%B5_%D0%B2%D1%80%D0%B5%D0%BC%D1%8F" title="Международное атомное время – Russian" lang="ru" hreflang="ru" data-title="Международное атомное время" data-language-autonym="Русский" data-language-local-name="Russian" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-sk mw-list-item"><a href="https://sk.wikipedia.org/wiki/Medzin%C3%A1rodn%C3%BD_at%C3%B3mov%C3%BD_%C4%8Das" title="Medzinárodný atómový čas – Slovak" lang="sk" hreflang="sk" data-title="Medzinárodný atómový čas" data-language-autonym="Slovenčina" data-language-local-name="Slovak" class="interlanguage-link-target"><span>Slovenčina</span></a></li><li class="interlanguage-link interwiki-sl mw-list-item"><a href="https://sl.wikipedia.org/wiki/Mednarodni_atomski_%C4%8Das" title="Mednarodni atomski čas – Slovenian" lang="sl" hreflang="sl" data-title="Mednarodni atomski čas" data-language-autonym="Slovenščina" data-language-local-name="Slovenian" class="interlanguage-link-target"><span>Slovenščina</span></a></li><li class="interlanguage-link interwiki-sv mw-list-item"><a href="https://sv.wikipedia.org/wiki/Internationell_atomtid" title="Internationell atomtid – Swedish" lang="sv" hreflang="sv" data-title="Internationell atomtid" data-language-autonym="Svenska" data-language-local-name="Swedish" class="interlanguage-link-target"><span>Svenska</span></a></li><li class="interlanguage-link interwiki-th mw-list-item"><a href="https://th.wikipedia.org/wiki/%E0%B9%80%E0%B8%A7%E0%B8%A5%E0%B8%B2%E0%B8%AD%E0%B8%B0%E0%B8%95%E0%B8%AD%E0%B8%A1%E0%B8%A3%E0%B8%B0%E0%B8%AB%E0%B8%A7%E0%B9%88%E0%B8%B2%E0%B8%87%E0%B8%9B%E0%B8%A3%E0%B8%B0%E0%B9%80%E0%B8%97%E0%B8%A8" title="เวลาอะตอมระหว่างประเทศ – Thai" lang="th" hreflang="th" data-title="เวลาอะตอมระหว่างประเทศ" data-language-autonym="ไทย" data-language-local-name="Thai" class="interlanguage-link-target"><span>ไทย</span></a></li><li class="interlanguage-link interwiki-uk mw-list-item"><a href="https://uk.wikipedia.org/wiki/%D0%9C%D1%96%D0%B6%D0%BD%D0%B0%D1%80%D0%BE%D0%B4%D0%BD%D0%B8%D0%B9_%D0%B0%D1%82%D0%BE%D0%BC%D0%BD%D0%B8%D0%B9_%D1%87%D0%B0%D1%81" title="Міжнародний атомний час – Ukrainian" lang="uk" hreflang="uk" data-title="Міжнародний атомний час" data-language-autonym="Українська" data-language-local-name="Ukrainian" class="interlanguage-link-target"><span>Українська</span></a></li><li class="interlanguage-link interwiki-ur mw-list-item"><a href="https://ur.wikipedia.org/wiki/%D8%B9%D8%A7%D9%84%D9%85%DB%8C_%D8%AC%D9%88%DB%81%D8%B1%DB%8C_%D9%88%D9%82%D8%AA" title="عالمی جوہری وقت – Urdu" lang="ur" hreflang="ur" data-title="عالمی جوہری وقت" data-language-autonym="اردو" data-language-local-name="Urdu" 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dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Time standard based on atomic clocks</div> <p class="mw-empty-elt"> </p><p><b>International Atomic Time</b> (abbreviated <b>TAI</b>, from its French name <b><span title="French-language text"><i lang="fr">temps atomique international</i></span></b><sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>) is a high-precision <a href="/wiki/Atomic_clock" title="Atomic clock">atomic</a> <a href="/wiki/Coordinate_time" title="Coordinate time">coordinate</a> <a href="/wiki/Time_standard" title="Time standard">time standard</a> based on the notional passage of <a href="/wiki/Proper_time" title="Proper time">proper time</a> on Earth's <a href="/wiki/Geoid" title="Geoid">geoid</a>.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> TAI is a <a href="/wiki/Weighted_average" class="mw-redirect" title="Weighted average">weighted average</a> of the time kept by over 450 <a href="/wiki/Atomic_clock" title="Atomic clock">atomic clocks</a> in over 80 national laboratories worldwide.<sup id="cite_ref-Time_n.d._3-0" class="reference"><a href="#cite_note-Time_n.d.-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> It is a continuous scale of time, without <a href="/wiki/Leap_seconds" class="mw-redirect" title="Leap seconds">leap seconds</a>, and it is the principal realisation of <a href="/wiki/Terrestrial_Time" title="Terrestrial Time">Terrestrial Time</a> (with a fixed offset of <a href="/wiki/Epoch_(astronomy)" title="Epoch (astronomy)">epoch</a>). It is the basis for <a href="/wiki/Coordinated_Universal_Time" title="Coordinated Universal Time">Coordinated Universal Time</a> (UTC), which is used for civil timekeeping all over the Earth's surface and which has leap seconds. </p><p>UTC deviates from TAI by a number of whole seconds. As of 1 January 2017<sup class="plainlinks noexcerpt noprint asof-tag update" style="display:none;"><a class="external text" href="https://en.wikipedia.org/w/index.php?title=International_Atomic_Time&action=edit">[update]</a></sup>, immediately after the most recent <a href="/wiki/Leap_second" title="Leap second">leap second</a> was put into effect,<sup id="cite_ref-Bulletin_C_52_4-0" class="reference"><a href="#cite_note-Bulletin_C_52-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> UTC has been exactly 37 seconds behind TAI. The 37 seconds result from the initial difference of 10 seconds at the start of 1972, plus 27 leap seconds in UTC since 1972. In 2022, the <a href="/wiki/General_Conference_on_Weights_and_Measures" title="General Conference on Weights and Measures">General Conference on Weights and Measures</a> decided to abandon the leap second by or before 2035, at which point the difference between TAI and UTC will remain fixed.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> </p><p>TAI may be reported using traditional means of specifying days, carried over from non-uniform time standards based on the rotation of the Earth. Specifically, both <a href="/wiki/Julian_day" title="Julian day">Julian days</a> and the <a href="/wiki/Gregorian_calendar" title="Gregorian calendar">Gregorian calendar</a> are used. TAI in this form was synchronised with <a href="/wiki/Universal_Time" title="Universal Time">Universal Time</a> at the beginning of 1958, and the two have drifted apart ever since, due primarily to the slowing rotation of the Earth. </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Operation">Operation</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=International_Atomic_Time&action=edit&section=1" title="Edit section: Operation"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>TAI is a <a href="/wiki/Weighted_average" class="mw-redirect" title="Weighted average">weighted average</a> of the time kept by over 450 <a href="/wiki/Atomic_clock" title="Atomic clock">atomic clocks</a> in over 80 national laboratories worldwide.<sup id="cite_ref-Time_n.d._3-1" class="reference"><a href="#cite_note-Time_n.d.-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> The majority of the clocks involved are <a href="/wiki/Caesium_clock" class="mw-redirect" title="Caesium clock">caesium clocks</a>; the <a href="/wiki/International_System_of_Units" title="International System of Units">International System of Units</a> (SI) definition of the <a href="/wiki/Second" title="Second">second</a> is based on <a href="/wiki/Caesium" title="Caesium">caesium</a>.<sup id="cite_ref-FOOTNOTEMcCarthySeidelmann2009207,_214_6-0" class="reference"><a href="#cite_note-FOOTNOTEMcCarthySeidelmann2009207,_214-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> The clocks are compared using <a href="/wiki/GPS" class="mw-redirect" title="GPS">GPS</a> signals and <a href="/wiki/Two-way_satellite_time_and_frequency_transfer" title="Two-way satellite time and frequency transfer">two-way satellite time and frequency transfer</a>.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> Due to the <a href="/wiki/Signal_averaging" title="Signal averaging">signal averaging</a> TAI is an <a href="/wiki/Order_of_magnitude" title="Order of magnitude">order of magnitude</a> more stable than its best constituent clock. </p><p>The participating institutions each broadcast, in <a href="/wiki/Real-time_data" title="Real-time data">real time</a>, a frequency signal with <a href="/wiki/Timecode" title="Timecode">timecodes</a>, which is their estimate of TAI. Time codes are usually published in the form of UTC, which differs from TAI by a well-known integer number of seconds. These time scales are denoted in the form <i>UTC(NPL)</i> in the UTC form, where <i>NPL</i> here identifies the <a href="/wiki/National_Physical_Laboratory,_UK" class="mw-redirect" title="National Physical Laboratory, UK">National Physical Laboratory, UK</a>. The TAI form may be denoted <i>TAI(NPL)</i>. The latter is not to be confused with <i>TA(NPL)</i>, which denotes an independent atomic time scale, not synchronised to TAI or to anything else. </p><p>The clocks at different institutions are regularly compared against each other. The <a href="/wiki/International_Bureau_of_Weights_and_Measures" title="International Bureau of Weights and Measures">International Bureau of Weights and Measures</a> (BIPM, France), combines these measurements to retrospectively calculate the weighted average that forms the most stable time scale possible.<sup id="cite_ref-Time_n.d._3-2" class="reference"><a href="#cite_note-Time_n.d.-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> This combined time scale is published monthly in "Circular T",<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> and is the <a href="/wiki/Canonical_form" title="Canonical form">canonical</a> TAI. This time scale is expressed in the form of tables of differences UTC − UTC(<i>k</i>) (equal to TAI − TAI(<i>k</i>)) for each participating institution <i>k</i>. The same circular also gives tables of TAI − TA(<i>k</i>), for the various unsynchronised atomic time scales. </p><p>Errors in publication may be corrected by issuing a revision of the faulty Circular T or by errata in a subsequent Circular T. Aside from this, once published in Circular T, the TAI scale is not revised. In hindsight, it is possible to discover errors in TAI and to make better estimates of the true proper time scale. Since the published circulars are definitive, better estimates do not create another version of TAI; it is instead considered to be creating a better realisation of <a href="/wiki/Terrestrial_Time" title="Terrestrial Time">Terrestrial Time</a> (TT). </p> <div class="mw-heading mw-heading2"><h2 id="History">History</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=International_Atomic_Time&action=edit&section=2" title="Edit section: History"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Early atomic time scales consisted of <a href="/wiki/Quartz_clock" title="Quartz clock">quartz clocks</a> with frequencies calibrated by a single atomic clock; the atomic clocks were not operated continuously. Atomic timekeeping services started experimentally in 1955, using the first caesium atomic clock at the <a href="/wiki/National_Physical_Laboratory_(United_Kingdom)" title="National Physical Laboratory (United Kingdom)">National Physical Laboratory, UK (NPL)</a>. It was used as a basis for calibrating the quartz clocks at the <a href="/wiki/Royal_Greenwich_Observatory" class="mw-redirect" title="Royal Greenwich Observatory">Royal Greenwich Observatory</a> and to establish a time scale, called Greenwich Atomic (GA). The <a href="/wiki/United_States_Naval_Observatory" title="United States Naval Observatory">United States Naval Observatory</a> began the A.1 scale on 13 September 1956, using an <a href="/wiki/Atomichron" title="Atomichron">Atomichron</a> commercial atomic clock, followed by the NBS-A scale at the <a href="/wiki/National_Bureau_of_Standards" class="mw-redirect" title="National Bureau of Standards">National Bureau of Standards</a>, <a href="/wiki/Boulder,_Colorado" title="Boulder, Colorado">Boulder, Colorado</a> on 9 October 1957.<sup id="cite_ref-FOOTNOTEMcCarthySeidelmann2009199–200_9-0" class="reference"><a href="#cite_note-FOOTNOTEMcCarthySeidelmann2009199–200-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> </p><p>The <a href="/wiki/International_Time_Bureau" title="International Time Bureau">International Time Bureau</a> (BIH) began a time scale, T<sub>m</sub> or AM, in July 1955, using both local caesium clocks and comparisons to distant clocks using the phase of <a href="/wiki/VLF" class="mw-redirect" title="VLF">VLF</a> radio signals. The BIH scale, A.1, and NBS-A were defined by an <a href="/wiki/Epoch" title="Epoch">epoch</a> at the beginning of 1958<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>a<span class="cite-bracket">]</span></a></sup> The procedures used by the BIH evolved, and the name for the time scale changed: <i>A3</i> in 1964<sup id="cite_ref-12" class="reference"><a href="#cite_note-12"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> and <i>TA(BIH)</i> in 1969.<sup id="cite_ref-FOOTNOTEMcCarthySeidelmann2009200–201_13-0" class="reference"><a href="#cite_note-FOOTNOTEMcCarthySeidelmann2009200–201-13"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> </p><p>The SI second was defined in terms of the caesium atom in 1967. From 1971 to 1975 the <a href="/wiki/General_Conference_on_Weights_and_Measures" title="General Conference on Weights and Measures">General Conference on Weights and Measures</a> and the <a href="/wiki/International_Committee_for_Weights_and_Measures" class="mw-redirect" title="International Committee for Weights and Measures">International Committee for Weights and Measures</a> made a series of decisions that designated the BIPM time scale International Atomic Time (TAI).<sup id="cite_ref-FOOTNOTEMcCarthySeidelmann2009203–204_14-0" class="reference"><a href="#cite_note-FOOTNOTEMcCarthySeidelmann2009203–204-14"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup> </p><p>In the 1970s, it became clear that the clocks participating in TAI were ticking at different rates due to <a href="/wiki/Gravitational_time_dilation" title="Gravitational time dilation">gravitational time dilation</a>, and the combined TAI scale, therefore, corresponded to an average of the altitudes of the various clocks. Starting from the Julian Date 2443144.5 (1 January 1977 00:00:00 TAI), corrections were applied to the output of all participating clocks, so that TAI would correspond to proper time at the <a href="/wiki/Geoid" title="Geoid">geoid</a> (<a href="/wiki/Mean_sea_level" class="mw-redirect" title="Mean sea level">mean sea level</a>). Because the clocks were, on average, well above sea level, this meant that TAI slowed by about one part in a trillion. The former uncorrected time scale continues to be published under the name <i>EAL</i> (<i>Échelle Atomique Libre</i>, meaning <i>Free Atomic Scale</i>).<sup id="cite_ref-FOOTNOTEMcCarthySeidelmann2009215_15-0" class="reference"><a href="#cite_note-FOOTNOTEMcCarthySeidelmann2009215-15"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup> </p><p>The instant that the gravitational correction started to be applied serves as the epoch for <a href="/wiki/Barycentric_Coordinate_Time" title="Barycentric Coordinate Time">Barycentric Coordinate Time</a> (TCB), <a href="/wiki/Geocentric_Coordinate_Time" title="Geocentric Coordinate Time">Geocentric Coordinate Time</a> (TCG), and <a href="/wiki/Terrestrial_Time" title="Terrestrial Time">Terrestrial Time</a> (TT), which represent three fundamental time scales in the solar system.<sup id="cite_ref-16" class="reference"><a href="#cite_note-16"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup> All three of these time scales were defined to read JD 2443144.5003725 (1 January 1977 00:00:32.184) exactly at that instant.<sup id="cite_ref-17" class="reference"><a href="#cite_note-17"><span class="cite-bracket">[</span>b<span class="cite-bracket">]</span></a></sup> TAI was henceforth a realisation of TT, with the equation TT(TAI) = TAI + 32.184 s.<sup id="cite_ref-FOOTNOTEMcCarthySeidelmann2009218&ndash;219_18-0" class="reference"><a href="#cite_note-FOOTNOTEMcCarthySeidelmann2009218&ndash;219-18"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> </p><p>The continued existence of TAI was questioned in a 2007 letter from the BIPM to the ITU-R which stated, "In the case of a redefinition of UTC without leap seconds, the CCTF would consider discussing the possibility of suppressing TAI, as it would remain parallel to the continuous UTC."<sup id="cite_ref-19" class="reference"><a href="#cite_note-19"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Relation_to_UTC">Relation to UTC</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=International_Atomic_Time&action=edit&section=3" title="Edit section: Relation to UTC"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Contrary to TAI, UTC is a <a href="/wiki/Discontinuous" class="mw-redirect" title="Discontinuous">discontinuous</a> time scale. It is occasionally adjusted by leap seconds. Between these adjustments, it is composed of segments that are mapped to atomic time by a constant offset. From its beginning in 1961 through December 1971, the adjustments were made regularly in fractional leap seconds so that UTC approximated <a href="/wiki/UT2" class="mw-redirect" title="UT2">UT2</a>. Afterwards, these adjustments were made only in whole seconds to approximate <a href="/wiki/UT1" class="mw-redirect" title="UT1">UT1</a>. This was a compromise arrangement in order to enable a publicly broadcast time scale. The less frequent whole-second adjustments meant that the time scale would be more stable and easier to synchronize internationally. The fact that it continues to approximate UT1 means that tasks such as <a href="/wiki/Navigation" title="Navigation">navigation</a> which require a source of Universal Time continue to be well served by the public broadcast of UTC.<sup id="cite_ref-FOOTNOTEMcCarthySeidelmann2009227&ndash;229_20-0" class="reference"><a href="#cite_note-FOOTNOTEMcCarthySeidelmann2009227&ndash;229-20"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=International_Atomic_Time&action=edit&section=4" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Clock_synchronization" title="Clock synchronization">Clock synchronization</a></li> <li><a href="/wiki/Time_and_frequency_transfer" title="Time and frequency transfer">Time and frequency transfer</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="Notes">Notes</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=International_Atomic_Time&action=edit&section=5" title="Edit section: Notes"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist reflist-lower-alpha"> <div class="mw-references-wrap"><ol class="references"> <li id="cite_note-11"><span class="mw-cite-backlink"><b><a href="#cite_ref-11">^</a></b></span> <span class="reference-text">They were set to read Julian Date 2436204.5 (1 January 1958 00:00:00) at the corresponding <a href="/wiki/UT2" class="mw-redirect" title="UT2">UT2</a> instant. However, each observatory used its own value of UT2.<sup id="cite_ref-FOOTNOTEGuinot2000181_10-0" class="reference"><a href="#cite_note-FOOTNOTEGuinot2000181-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup></span> </li> <li id="cite_note-17"><span class="mw-cite-backlink"><b><a href="#cite_ref-17">^</a></b></span> <span class="reference-text">The 32.184 second offset is to provide continuity with the older <a href="/wiki/Ephemeris_time" title="Ephemeris time">ephemeris time</a>.</span> </li> </ol></div></div> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=International_Atomic_Time&action=edit&section=6" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239549316">.mw-parser-output .refbegin{margin-bottom:0.5em}.mw-parser-output .refbegin-hanging-indents>ul{margin-left:0}.mw-parser-output .refbegin-hanging-indents>ul>li{margin-left:0;padding-left:3.2em;text-indent:-3.2em}.mw-parser-output .refbegin-hanging-indents ul,.mw-parser-output .refbegin-hanging-indents ul li{list-style:none}@media(max-width:720px){.mw-parser-output .refbegin-hanging-indents>ul>li{padding-left:1.6em;text-indent:-1.6em}}.mw-parser-output .refbegin-columns{margin-top:0.3em}.mw-parser-output .refbegin-columns ul{margin-top:0}.mw-parser-output .refbegin-columns li{page-break-inside:avoid;break-inside:avoid-column}@media screen{.mw-parser-output .refbegin{font-size:90%}}</style><div class="refbegin" style=""> <ul><li><style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20191019051734/http://maia.usno.navy.mil/ser7/tai-utc.dat">"History of TAI−UTC"</a>. Time Service Dept., United States Naval Observatory. 2009. Archived from <a rel="nofollow" class="external text" href="http://maia.usno.navy.mil/ser7/tai-utc.dat">the original</a> on 19 October 2019<span class="reference-accessdate">. Retrieved <span class="nowrap">4 January</span> 2010</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=History+of+TAI%E2%88%92UTC&rft.pub=Time+Service+Dept.%2C+United+States+Naval+Observatory&rft.date=2009&rft_id=http%3A%2F%2Fmaia.usno.navy.mil%2Fser7%2Ftai-utc.dat&rfr_id=info%3Asid%2Fen.wikipedia.org%3AInternational+Atomic+Time" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.bipm.org/en/scientific/tai/tai.html">"International Atomic Time"</a>. <a href="/wiki/International_Bureau_of_Weights_and_Measures" title="International Bureau of Weights and Measures">International Bureau of Weights and Measures</a>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20110110113226/http://www.bipm.org/en/scientific/tai/tai.html">Archived</a> from the original on 10 January 2011<span class="reference-accessdate">. Retrieved <span class="nowrap">24 January</span> 2020</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=International+Atomic+Time&rft.pub=International+Bureau+of+Weights+and+Measures&rft_id=http%3A%2F%2Fwww.bipm.org%2Fen%2Fscientific%2Ftai%2Ftai.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3AInternational+Atomic+Time" class="Z3988"></span></li></ul> </div> <div class="mw-heading mw-heading3"><h3 id="Footnotes">Footnotes</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=International_Atomic_Time&action=edit&section=7" title="Edit section: Footnotes"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1239543626"><div class="reflist"> <div class="mw-references-wrap mw-references-columns"><ol class="references"> <li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text">Temps atomique 1975<sup class="noprint Inline-Template" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Please_clarify" title="Wikipedia:Please clarify"><span title="The text near this tag needs further explanation. (April 2020)">further explanation needed</span></a></i>]</sup></span> </li> <li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFGuinot1986" class="citation journal cs1">Guinot, B. (1986). "Is the International Atomic Time TAI a coordinate time or a proper time?". <i>Celestial Mechanics</i>. <b>38</b> (2): 155–161. <a href="/wiki/Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/1986CeMec..38..155G">1986CeMec..38..155G</a>. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1007%2FBF01230427">10.1007/BF01230427</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:120564915">120564915</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Celestial+Mechanics&rft.atitle=Is+the+International+Atomic+Time+TAI+a+coordinate+time+or+a+proper+time%3F&rft.volume=38&rft.issue=2&rft.pages=155-161&rft.date=1986&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A120564915%23id-name%3DS2CID&rft_id=info%3Adoi%2F10.1007%2FBF01230427&rft_id=info%3Abibcode%2F1986CeMec..38..155G&rft.aulast=Guinot&rft.aufirst=B.&rfr_id=info%3Asid%2Fen.wikipedia.org%3AInternational+Atomic+Time" class="Z3988"></span></span> </li> <li id="cite_note-Time_n.d.-3"><span class="mw-cite-backlink">^ <a href="#cite_ref-Time_n.d._3-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Time_n.d._3-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-Time_n.d._3-2"><sup><i><b>c</b></i></sup></a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation book cs1"><a rel="nofollow" class="external text" href="https://webtai.bipm.org/ftp/pub/tai/annual-reports/bipm-annual-report/annual_report_2020.pdf"><i>BIPM Annual Report on Time Activities</i></a> <span class="cs1-format">(PDF)</span>. Vol. 15. International Bureau of Weights and Measures. 2020. p. 9. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-92-822-2280-5" title="Special:BookSources/978-92-822-2280-5"><bdi>978-92-822-2280-5</bdi></a>. <a href="/wiki/ISSN_(identifier)" class="mw-redirect" title="ISSN (identifier)">ISSN</a> <a rel="nofollow" class="external text" href="https://search.worldcat.org/issn/1994-9405">1994-9405</a>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20210814134522/https://webtai.bipm.org/ftp/pub/tai/annual-reports/bipm-annual-report/annual_report_2020.pdf">Archived</a> <span class="cs1-format">(PDF)</span> from the original on 14 August 2021<span class="reference-accessdate">. Retrieved <span class="nowrap">16 June</span> 2022</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=BIPM+Annual+Report+on+Time+Activities&rft.pages=9&rft.pub=International+Bureau+of+Weights+and+Measures&rft.date=2020&rft.issn=1994-9405&rft.isbn=978-92-822-2280-5&rft_id=https%3A%2F%2Fwebtai.bipm.org%2Fftp%2Fpub%2Ftai%2Fannual-reports%2Fbipm-annual-report%2Fannual_report_2020.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3AInternational+Atomic+Time" class="Z3988"></span></span> </li> <li id="cite_note-Bulletin_C_52-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-Bulletin_C_52_4-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFBizouard2016" class="citation web cs1">Bizouard, Christian (6 July 2016). <a rel="nofollow" class="external text" href="http://hpiers.obspm.fr/eoppc/bul/bulc/bulletinc.52">"Bulletin C 52"</a>. Paris: <a href="/wiki/IERS" class="mw-redirect" title="IERS">IERS</a>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20170813224025/https://hpiers.obspm.fr/eoppc/bul/bulc/bulletinc.52">Archived</a> from the original on 13 August 2017<span class="reference-accessdate">. Retrieved <span class="nowrap">31 December</span> 2016</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Bulletin+C+52&rft.place=Paris&rft.pub=IERS&rft.date=2016-07-06&rft.aulast=Bizouard&rft.aufirst=Christian&rft_id=http%3A%2F%2Fhpiers.obspm.fr%2Feoppc%2Fbul%2Fbulc%2Fbulletinc.52&rfr_id=info%3Asid%2Fen.wikipedia.org%3AInternational+Atomic+Time" class="Z3988"></span></span> </li> <li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFAgence_France-Presse2022" class="citation news cs1">Agence France-Presse (18 November 2022). <a rel="nofollow" class="external text" href="https://www.theguardian.com/world/2022/nov/18/do-not-adjust-your-clock-scientists-call-time-on-the-leap-second">"Do not adjust your clock: scientists call time on the leap second"</a>. <i>The Guardian</i>. <a href="/wiki/ISSN_(identifier)" class="mw-redirect" title="ISSN (identifier)">ISSN</a> <a rel="nofollow" class="external text" href="https://search.worldcat.org/issn/0261-3077">0261-3077</a><span class="reference-accessdate">. Retrieved <span class="nowrap">23 October</span> 2024</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=The+Guardian&rft.atitle=Do+not+adjust+your+clock%3A+scientists+call+time+on+the+leap+second&rft.date=2022-11-18&rft.issn=0261-3077&rft.au=Agence+France-Presse&rft_id=https%3A%2F%2Fwww.theguardian.com%2Fworld%2F2022%2Fnov%2F18%2Fdo-not-adjust-your-clock-scientists-call-time-on-the-leap-second&rfr_id=info%3Asid%2Fen.wikipedia.org%3AInternational+Atomic+Time" class="Z3988"></span></span> </li> <li id="cite_note-FOOTNOTEMcCarthySeidelmann2009207,_214-6"><span class="mw-cite-backlink"><b><a href="#cite_ref-FOOTNOTEMcCarthySeidelmann2009207,_214_6-0">^</a></b></span> <span class="reference-text"><a href="#CITEREFMcCarthySeidelmann2009">McCarthy & Seidelmann 2009</a>, p. 207, 214.</span> </li> <li id="cite_note-7"><span class="mw-cite-backlink"><b><a href="#cite_ref-7">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation cs2"><a rel="nofollow" class="external text" href="https://webtai.bipm.org/ftp/pub/tai/other-products/notes/explanatory_supplement_v0.6.pdf"><i>Explanatory Supplement of BIPM Circular T</i></a> <span class="cs1-format">(PDF)</span>, <a href="/wiki/International_Bureau_of_Weights_and_Measures" title="International Bureau of Weights and Measures">International Bureau of Weights and Measures</a>, 12 July 2021, <a rel="nofollow" class="external text" href="https://ghostarchive.org/archive/20221009/https://webtai.bipm.org/ftp/pub/tai/other-products/notes/explanatory_supplement_v0.6.pdf">archived</a> <span class="cs1-format">(PDF)</span> from the original on 9 October 2022<span class="reference-accessdate">, retrieved <span class="nowrap">16 June</span> 2022</span></cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Explanatory+Supplement+of+BIPM+Circular+T&rft.pub=International+Bureau+of+Weights+and+Measures&rft.date=2021-07-12&rft_id=https%3A%2F%2Fwebtai.bipm.org%2Fftp%2Fpub%2Ftai%2Fother-products%2Fnotes%2Fexplanatory_supplement_v0.6.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3AInternational+Atomic+Time" class="Z3988"></span></span> </li> <li id="cite_note-8"><span class="mw-cite-backlink"><b><a href="#cite_ref-8">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation cs2"><a rel="nofollow" class="external text" href="https://www.bipm.org/en/time-ftp/circular-t"><i>Circular T</i></a>, <a href="/wiki/International_Bureau_of_Weights_and_Measures" title="International Bureau of Weights and Measures">International Bureau of Weights and Measures</a><span class="reference-accessdate">, retrieved <span class="nowrap">16 June</span> 2022</span></cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Circular+T&rft.pub=International+Bureau+of+Weights+and+Measures&rft_id=https%3A%2F%2Fwww.bipm.org%2Fen%2Ftime-ftp%2Fcircular-t&rfr_id=info%3Asid%2Fen.wikipedia.org%3AInternational+Atomic+Time" class="Z3988"></span></span> </li> <li id="cite_note-FOOTNOTEMcCarthySeidelmann2009199–200-9"><span class="mw-cite-backlink"><b><a href="#cite_ref-FOOTNOTEMcCarthySeidelmann2009199–200_9-0">^</a></b></span> <span class="reference-text"><a href="#CITEREFMcCarthySeidelmann2009">McCarthy & Seidelmann 2009</a>, pp. 199–200.</span> </li> <li id="cite_note-FOOTNOTEGuinot2000181-10"><span class="mw-cite-backlink"><b><a href="#cite_ref-FOOTNOTEGuinot2000181_10-0">^</a></b></span> <span class="reference-text"><a href="#CITEREFGuinot2000">Guinot 2000</a>, p. 181.</span> </li> <li id="cite_note-12"><span class="mw-cite-backlink"><b><a href="#cite_ref-12">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFAllen" class="citation web cs1">Allen, Steve. <a rel="nofollow" class="external text" href="https://www.ucolick.org/~sla/leapsecs/taiepoch.html">"The epoch of TAI is 1961-01-01T20:00:00 UT2"</a>. UCO/Lick Observatory. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20211010014537/https://www.ucolick.org/~sla/leapsecs/taiepoch.html">Archived</a> from the original on 10 October 2021<span class="reference-accessdate">. Retrieved <span class="nowrap">21 January</span> 2019</span>. <q>By 1964 BIH realized that some atomic chronometers were much better than others, and they constructed A3 based on the best 3</q></cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=The+epoch+of+TAI+is+1961-01-01T20%3A00%3A00+UT2&rft.pub=UCO%2FLick+Observatory&rft.aulast=Allen&rft.aufirst=Steve&rft_id=https%3A%2F%2Fwww.ucolick.org%2F~sla%2Fleapsecs%2Ftaiepoch.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3AInternational+Atomic+Time" class="Z3988"></span></span> </li> <li id="cite_note-FOOTNOTEMcCarthySeidelmann2009200–201-13"><span class="mw-cite-backlink"><b><a href="#cite_ref-FOOTNOTEMcCarthySeidelmann2009200–201_13-0">^</a></b></span> <span class="reference-text"><a href="#CITEREFMcCarthySeidelmann2009">McCarthy & Seidelmann 2009</a>, pp. 200–201.</span> </li> <li id="cite_note-FOOTNOTEMcCarthySeidelmann2009203–204-14"><span class="mw-cite-backlink"><b><a href="#cite_ref-FOOTNOTEMcCarthySeidelmann2009203–204_14-0">^</a></b></span> <span class="reference-text"><a href="#CITEREFMcCarthySeidelmann2009">McCarthy & Seidelmann 2009</a>, pp. 203–204.</span> </li> <li id="cite_note-FOOTNOTEMcCarthySeidelmann2009215-15"><span class="mw-cite-backlink"><b><a href="#cite_ref-FOOTNOTEMcCarthySeidelmann2009215_15-0">^</a></b></span> <span class="reference-text"><a href="#CITEREFMcCarthySeidelmann2009">McCarthy & Seidelmann 2009</a>, p. 215.</span> </li> <li id="cite_note-16"><span class="mw-cite-backlink"><b><a href="#cite_ref-16">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFBrumbergKopeikin1990" class="citation journal cs1">Brumberg, V.A.; Kopeikin, S.M. (March 1990). "Relativistic time scales in the solar system". <i><a href="/wiki/Celestial_Mechanics_and_Dynamical_Astronomy" title="Celestial Mechanics and Dynamical Astronomy">Celestial Mechanics and Dynamical Astronomy</a></i>. <b>48</b> (1): 23–44. <a href="/wiki/Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/1990CeMDA..48...23B">1990CeMDA..48...23B</a>. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1007%2FBF00050674">10.1007/BF00050674</a>. <a href="/wiki/ISSN_(identifier)" class="mw-redirect" title="ISSN (identifier)">ISSN</a> <a rel="nofollow" class="external text" href="https://search.worldcat.org/issn/0923-2958">0923-2958</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:120112678">120112678</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Celestial+Mechanics+and+Dynamical+Astronomy&rft.atitle=Relativistic+time+scales+in+the+solar+system&rft.volume=48&rft.issue=1&rft.pages=23-44&rft.date=1990-03&rft_id=info%3Adoi%2F10.1007%2FBF00050674&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A120112678%23id-name%3DS2CID&rft.issn=0923-2958&rft_id=info%3Abibcode%2F1990CeMDA..48...23B&rft.aulast=Brumberg&rft.aufirst=V.A.&rft.au=Kopeikin%2C+S.M.&rfr_id=info%3Asid%2Fen.wikipedia.org%3AInternational+Atomic+Time" class="Z3988"></span></span> </li> <li id="cite_note-FOOTNOTEMcCarthySeidelmann2009218&ndash;219-18"><span class="mw-cite-backlink"><b><a href="#cite_ref-FOOTNOTEMcCarthySeidelmann2009218&ndash;219_18-0">^</a></b></span> <span class="reference-text"><a href="#CITEREFMcCarthySeidelmann2009">McCarthy & Seidelmann 2009</a>, p. 218–219.</span> </li> <li id="cite_note-19"><span class="mw-cite-backlink"><b><a href="#cite_ref-19">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20120316100907/http://www.bipm.org/cc/CCTF/Allowed/18/CCTF_09-27_note_on_UTC-ITU-R.pdf">"CCTF 09-27"</a> <span class="cs1-format">(PDF)</span>. International Bureau of Weights and Measures. 3 September 2007. Archived from <a rel="nofollow" class="external text" href="http://www.bipm.org/cc/CCTF/Allowed/18/CCTF_09-27_note_on_UTC-ITU-R.pdf">the original</a> <span class="cs1-format">(PDF)</span> on 16 March 2012<span class="reference-accessdate">. Retrieved <span class="nowrap">24 September</span> 2018</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=CCTF+09-27&rft.pub=International+Bureau+of+Weights+and+Measures&rft.date=2007-09-03&rft_id=http%3A%2F%2Fwww.bipm.org%2Fcc%2FCCTF%2FAllowed%2F18%2FCCTF_09-27_note_on_UTC-ITU-R.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3AInternational+Atomic+Time" class="Z3988"></span></span> </li> <li id="cite_note-FOOTNOTEMcCarthySeidelmann2009227&ndash;229-20"><span class="mw-cite-backlink"><b><a href="#cite_ref-FOOTNOTEMcCarthySeidelmann2009227&ndash;229_20-0">^</a></b></span> <span class="reference-text"><a href="#CITEREFMcCarthySeidelmann2009">McCarthy & Seidelmann 2009</a>, p. 227–229.</span> </li> </ol></div></div> <div class="mw-heading mw-heading2"><h2 id="Bibliography">Bibliography</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=International_Atomic_Time&action=edit&section=8" title="Edit section: Bibliography"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1239549316"><div class="refbegin" style=""> <ul><li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFGuinot2000" class="citation book cs1">Guinot, B (2000). "History of the Bureau International de l'Heure". <a rel="nofollow" class="external text" href="https://books.google.com/books?id=pFDwAAAAMAAJ"><i>Polar Motion: Historical and Scientific Problems</i></a>. ASP Conference Series, Vol. 208. Vol. 208. p. 175. <a href="/wiki/Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/2000ASPC..208..175G">2000ASPC..208..175G</a>. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/1-58381-039-0" title="Special:BookSources/1-58381-039-0"><bdi>1-58381-039-0</bdi></a>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20220801225534/https://www.google.com/books/edition/Polar_Motion/pFDwAAAAMAAJ?hl=en">Archived</a> from the original on 1 August 2022.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=bookitem&rft.atitle=History+of+the+Bureau+International+de+l%27Heure&rft.btitle=Polar+Motion%3A+Historical+and+Scientific+Problems&rft.series=ASP+Conference+Series%2C+Vol.+208&rft.pages=175&rft.date=2000&rft_id=info%3Abibcode%2F2000ASPC..208..175G&rft.isbn=1-58381-039-0&rft.aulast=Guinot&rft.aufirst=B&rft_id=https%3A%2F%2Fbooks.google.com%2Fbooks%3Fid%3DpFDwAAAAMAAJ&rfr_id=info%3Asid%2Fen.wikipedia.org%3AInternational+Atomic+Time" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFMcCarthySeidelmann2009" class="citation book cs1"><a href="/wiki/Dennis_D._McCarthy" class="mw-redirect" title="Dennis D. McCarthy">McCarthy, Dennis D.</a>; Seidelmann, Kenneth P. (2009). <a rel="nofollow" class="external text" href="https://books.google.com/books?id=7NdrK4e77CIC"><i>Time: From Earth Rotation to Atomic Physics</i></a>. Weinheim: <a href="/wiki/Wiley-VCH_Verlag_GmbH_%26_Co._KGaA" class="mw-redirect" title="Wiley-VCH Verlag GmbH & Co. KGaA">Wiley-VCH Verlag GmbH & Co. KGaA</a>. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-3-527-40780-4" title="Special:BookSources/978-3-527-40780-4"><bdi>978-3-527-40780-4</bdi></a>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20210203233222/https://books.google.com/books?id=7NdrK4e77CIC">Archived</a> from the original on 3 February 2021<span class="reference-accessdate">. Retrieved <span class="nowrap">12 June</span> 2021</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Time%3A+From+Earth+Rotation+to+Atomic+Physics&rft.place=Weinheim&rft.pub=Wiley-VCH+Verlag+GmbH+%26+Co.+KGaA&rft.date=2009&rft.isbn=978-3-527-40780-4&rft.aulast=McCarthy&rft.aufirst=Dennis+D.&rft.au=Seidelmann%2C+Kenneth+P.&rft_id=https%3A%2F%2Fbooks.google.com%2Fbooks%3Fid%3D7NdrK4e77CIC&rfr_id=info%3Asid%2Fen.wikipedia.org%3AInternational+Atomic+Time" class="Z3988"></span></li></ul> </div> <div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=International_Atomic_Time&action=edit&section=9" title="Edit section: External links"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a rel="nofollow" class="external text" href="https://www.bipm.org/en/time-metrology">BIPM technical services: Time Metrology</a></li> <li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20060923185936/http://www.npl.co.uk/time/">Time and Frequency Section - National Physical Laboratory, UK</a></li> <li><a rel="nofollow" class="external text" href="http://hpiers.obspm.fr">IERS website</a></li> <li><a rel="nofollow" class="external text" href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/web-clock-faq">NIST Web Clock FAQs</a></li> <li><a rel="nofollow" class="external text" href="http://www.ucolick.org/~sla/leapsecs/timescales.html">History of time scales</a></li> <li><a rel="nofollow" class="external text" href="https://www.nist.gov/pml/div688/grp50/primary-frequency-standards.cfm">NIST-F1 Cesium Fountain Atomic Clock</a></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20090125051853/http://inms-ienm.nrc-cnrc.gc.ca/research/optical_frequency_projects_e.html#femtosecond">"Optical frequency comb for metrology and timekeeping"</a>. Archived from <a rel="nofollow" class="external text" href="http://inms-ienm.nrc-cnrc.gc.ca/research/optical_frequency_projects_e.html#femtosecond">the original</a> on 25 January 2009.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Optical+frequency+comb+for+metrology+and+timekeeping&rft_id=http%3A%2F%2Finms-ienm.nrc-cnrc.gc.ca%2Fresearch%2Foptical_frequency_projects_e.html%23femtosecond&rfr_id=info%3Asid%2Fen.wikipedia.org%3AInternational+Atomic+Time" class="Z3988"></span></li> <li><a rel="nofollow" class="external text" href="http://jjy.nict.go.jp/index-e.html">Japan Standard Time Project, NICT, Japan</a></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation cs2"><a rel="nofollow" class="external text" href="https://webtai.bipm.org/ftp/pub/tai/annual-reports/bipm-annual-report/TIMESERVICES/timeservices.pdf"><i>Time Dissemination Services</i></a> <span class="cs1-format">(PDF)</span>, Bureau International des Poids et Mesures, <a rel="nofollow" class="external text" href="https://ghostarchive.org/archive/20221009/https://webtai.bipm.org/ftp/pub/tai/annual-reports/bipm-annual-report/TIMESERVICES/timeservices.pdf">archived</a> <span class="cs1-format">(PDF)</span> from the original on 9 October 2022</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Time+Dissemination+Services&rft.pub=Bureau+International+des+Poids+et+Mesures&rft_id=https%3A%2F%2Fwebtai.bipm.org%2Fftp%2Fpub%2Ftai%2Fannual-reports%2Fbipm-annual-report%2FTIMESERVICES%2Ftimeservices.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3AInternational+Atomic+Time" class="Z3988"></span></li> <li><a rel="nofollow" class="external text" href="http://www.ipses.com/eng/In-depth-analysis/Standard-of-time-definition">Standard of time definition: UTC, GPS, LORAN and TAI</a></li></ul> <div class="navbox-styles"><style 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style="border-spacing:0;background:transparent;color:inherit"><tbody><tr><th scope="col" class="navbox-title" colspan="2"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1129693374"><style data-mw-deduplicate="TemplateStyles:r1239400231">.mw-parser-output .navbar{display:inline;font-size:88%;font-weight:normal}.mw-parser-output .navbar-collapse{float:left;text-align:left}.mw-parser-output .navbar-boxtext{word-spacing:0}.mw-parser-output .navbar ul{display:inline-block;white-space:nowrap;line-height:inherit}.mw-parser-output .navbar-brackets::before{margin-right:-0.125em;content:"[ "}.mw-parser-output .navbar-brackets::after{margin-left:-0.125em;content:" ]"}.mw-parser-output .navbar li{word-spacing:-0.125em}.mw-parser-output .navbar a>span,.mw-parser-output .navbar a>abbr{text-decoration:inherit}.mw-parser-output .navbar-mini abbr{font-variant:small-caps;border-bottom:none;text-decoration:none;cursor:inherit}.mw-parser-output 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href="/wiki/Time" title="Time">Time</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%">Key concepts</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Past" title="Past">Past</a></li> <li><a href="/wiki/Present" title="Present">Present</a></li> <li><a href="/wiki/Future" title="Future">Future</a></li> <li><a href="/wiki/Eternity" title="Eternity">Eternity</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Horology" class="mw-redirect" title="Horology">Measurement</a><br />and <a href="/wiki/Time_standard" title="Time standard">standards</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"></div><table class="nowraplinks navbox-subgroup" style="border-spacing:0"><tbody><tr><th scope="row" class="navbox-group" style="width:6.5em;font-weight:normal; text-align:center;"><a href="/wiki/Chronometry" title="Chronometry">Chronometry</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Coordinated_Universal_Time" title="Coordinated Universal Time">UTC</a></li> <li><a href="/wiki/Universal_Time" title="Universal Time">UT</a></li> <li><a class="mw-selflink selflink">TAI</a></li> <li><a href="/wiki/Unit_of_time" title="Unit of time">Unit of time</a></li> <li><a href="/wiki/Orders_of_magnitude_(time)" title="Orders of magnitude (time)">Orders of magnitude (time)</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:6.5em;font-weight:normal; text-align:center;"><a href="/wiki/System_of_measurement" class="mw-redirect" title="System of measurement">Measurement<br />systems</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Italian_six-hour_clock" title="Italian six-hour clock">Italian six-hour clock</a></li> <li><a href="/wiki/Thai_six-hour_clock" title="Thai six-hour clock">Thai six-hour clock</a></li> <li><a href="/wiki/12-hour_clock" title="12-hour clock">12-hour clock</a></li> <li><a href="/wiki/24-hour_clock" title="24-hour clock">24-hour clock</a></li> <li><a href="/wiki/Relative_hour" title="Relative hour">Relative hour</a></li> <li><a href="/wiki/Daylight_saving_time" title="Daylight saving time">Daylight saving time</a></li> <li><a href="/wiki/Traditional_Chinese_timekeeping" title="Traditional Chinese timekeeping">Chinese</a></li> <li><a href="/wiki/Decimal_time" title="Decimal time">Decimal</a></li> <li><a href="/wiki/Hexadecimal_time" title="Hexadecimal time">Hexadecimal</a></li> <li><a href="/wiki/Hindu_units_of_time" title="Hindu units of time">Hindu</a></li> <li><a href="/wiki/Metric_time" title="Metric time">Metric</a></li> <li><a href="/wiki/Roman_timekeeping" title="Roman timekeeping">Roman</a></li> <li><a href="/wiki/Sidereal_time" title="Sidereal time">Sidereal</a></li> <li><a href="/wiki/Solar_time" title="Solar time">Solar</a></li> <li><a href="/wiki/Time_zone" title="Time zone">Time zone</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:6.5em;font-weight:normal; text-align:center;"><a href="/wiki/Calendar" title="Calendar">Calendars</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Calendar#Systems" title="Calendar">Main types</a> <ul><li><a href="/wiki/Solar_calendar" title="Solar calendar">Solar</a></li> <li><a href="/wiki/Lunar_calendar" title="Lunar calendar">Lunar</a></li> <li><a href="/wiki/Lunisolar_calendar" title="Lunisolar calendar">Lunisolar</a></li></ul></li> <li><a href="/wiki/Gregorian_calendar" title="Gregorian calendar">Gregorian</a></li> <li><a href="/wiki/Julian_calendar" title="Julian calendar">Julian</a></li> <li><a href="/wiki/Hebrew_calendar" title="Hebrew calendar">Hebrew</a></li> <li><a href="/wiki/Islamic_calendar" title="Islamic calendar">Islamic</a></li> <li><a href="/wiki/Solar_Hijri_calendar" title="Solar Hijri calendar">Solar Hijri</a></li> <li><a href="/wiki/Chinese_calendar" title="Chinese calendar">Chinese</a></li> <li><a href="/wiki/Hindu_calendar" title="Hindu calendar">Hindu Panchang</a></li> <li><a href="/wiki/Maya_calendar" title="Maya calendar">Maya</a></li> <li><i><a href="/wiki/List_of_calendars" title="List of calendars">List</a></i></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:6.5em;font-weight:normal; text-align:center;"><a href="/wiki/Clock" title="Clock">Clocks</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Clock#Types" title="Clock">Main types</a> <ul><li><a href="/wiki/Astronomical_clock" title="Astronomical clock">astronomical</a> <ul><li><a href="/wiki/Astrarium" title="Astrarium">astrarium</a></li></ul></li> <li><a href="/wiki/Atomic_clock" title="Atomic clock">atomic</a> <ul><li><a href="/wiki/Quantum_clock" class="mw-redirect" title="Quantum clock">quantum</a></li></ul></li> <li><a href="/wiki/Hourglass" title="Hourglass">hourglass</a></li> <li><a href="/wiki/Marine_chronometer" title="Marine chronometer">marine</a></li> <li><a href="/wiki/Sundial" title="Sundial">sundial</a></li> <li><a href="/wiki/Watch" title="Watch">watch</a> <ul><li><a href="/wiki/Mechanical_watch" title="Mechanical watch">mechanical</a></li> <li><a href="/wiki/Stopwatch" title="Stopwatch">stopwatch</a></li></ul></li> <li><a href="/wiki/Water_clock" title="Water clock">water-based</a></li></ul></li> <li><a href="/wiki/Cuckoo_clock" title="Cuckoo clock">Cuckoo clock</a></li> <li><a href="/wiki/Digital_clock" title="Digital clock">Digital clock</a></li> <li><a href="/wiki/Grandfather_clock" title="Grandfather clock">Grandfather clock</a></li> <li><i><a href="/wiki/History_of_timekeeping_devices" title="History of timekeeping devices">History</a></i> <ul><li><i><a href="/wiki/Timeline_of_time_measurement_inventions" title="Timeline of time measurement inventions">Timeline</a></i></li></ul></li></ul> </div></td></tr></tbody></table><div></div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><div class="hlist"><ul><li><a href="/wiki/Chronology" title="Chronology">Chronology</a></li><li><a href="/wiki/History" title="History">History</a></li></ul></div></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Astronomical_chronology" title="Astronomical chronology">Astronomical chronology</a></li> <li><a href="/wiki/Big_History" title="Big History">Big History</a></li> <li><a href="/wiki/Calendar_era" title="Calendar era">Calendar era</a></li> <li><a href="/wiki/Deep_time" title="Deep time">Deep time</a></li> <li><a href="/wiki/Periodization" title="Periodization">Periodization</a></li> <li><a href="/wiki/Regnal_year" title="Regnal year">Regnal year</a></li> <li><a href="/wiki/Timeline" title="Timeline">Timeline</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Philosophy_of_space_and_time" title="Philosophy of space and time">Philosophy of time</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/A_series_and_B_series" title="A series and B series">A series and B series</a></li> <li><a href="/wiki/B-theory_of_time" title="B-theory of time">B-theory of time</a></li> <li><a href="/wiki/Chronocentrism" title="Chronocentrism">Chronocentrism</a></li> <li><a href="/wiki/Duration_(philosophy)" title="Duration (philosophy)">Duration</a></li> <li><a href="/wiki/Endurantism" title="Endurantism">Endurantism</a></li> <li><a href="/wiki/Eternal_return" title="Eternal return">Eternal return</a></li> <li><a href="/wiki/Eternalism_(philosophy_of_time)" title="Eternalism (philosophy of time)">Eternalism</a></li> <li><a href="/wiki/Event_(philosophy)" title="Event (philosophy)">Event</a></li> <li><a href="/wiki/Perdurantism" title="Perdurantism">Perdurantism</a></li> <li><a href="/wiki/Philosophical_presentism" title="Philosophical presentism">Presentism</a></li> <li><a href="/wiki/Temporal_finitism" title="Temporal finitism">Temporal finitism</a></li> <li><a href="/wiki/Temporal_parts" title="Temporal parts">Temporal parts</a></li> <li><i><a href="/wiki/The_Unreality_of_Time" title="The Unreality of Time">The Unreality of Time</a></i></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><div class="hlist"><ul><li><a href="/wiki/Category:Time_in_religion" title="Category:Time in religion">Religion</a></li><li><a href="/wiki/Template:Time_in_religion_and_mythology" title="Template:Time in religion and mythology">Mythology</a></li></ul></div></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Ages_of_Man" title="Ages of Man">Ages of Man</a></li> <li><a href="/wiki/Destiny" title="Destiny">Destiny</a></li> <li><a href="/wiki/Immortality" title="Immortality">Immortality</a></li> <li><a href="/wiki/The_Dreaming" title="The Dreaming">Dreamtime</a></li> <li><a href="/wiki/K%C4%81la" title="Kāla">Kāla</a></li> <li><a href="/wiki/Time_and_fate_deities" title="Time and fate deities">Time and fate deities</a> <ul><li><a href="/wiki/Father_Time" title="Father Time">Father Time</a></li></ul></li> <li><a href="/wiki/Wheel_of_time" title="Wheel of time">Wheel of time</a> <ul><li><a href="/wiki/Kalachakra" title="Kalachakra">Kalachakra</a></li></ul></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Time_perception" title="Time perception">Human experience</a><br />and <a href="/wiki/Time-use_research" title="Time-use research">use of time</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Chronemics" title="Chronemics">Chronemics</a></li> <li><a href="/wiki/Generation_time" title="Generation time">Generation time</a></li> <li><a href="/wiki/Mental_chronometry" title="Mental chronometry">Mental chronometry</a></li> <li><a href="/wiki/Duration_(music)" title="Duration (music)">Music</a> <ul><li><a href="/wiki/Tempo" title="Tempo">tempo</a></li> <li><a href="/wiki/Time_signature" title="Time signature">time signature</a></li></ul></li> <li><a href="/wiki/Rosy_retrospection" title="Rosy retrospection">Rosy retrospection</a></li> <li><a href="/wiki/Tense%E2%80%93aspect%E2%80%93mood" title="Tense–aspect–mood">Tense–aspect–mood</a></li> <li><a href="/wiki/Time_management" title="Time management">Time management</a></li> <li><a href="/wiki/Yesterday_(time)" title="Yesterday (time)">Yesterday</a> – <a href="/wiki/Present" title="Present">Today</a> – <a href="/wiki/Tomorrow_(time)" title="Tomorrow (time)">Tomorrow</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Time in <a href="/wiki/Science" title="Science">science</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"></div><table class="nowraplinks navbox-subgroup" style="border-spacing:0"><tbody><tr><th scope="row" class="navbox-group" style="width:6.5em;font-weight:normal; text-align:center;"><a href="/wiki/Geology" title="Geology">Geology</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Geologic_time_scale" title="Geologic time scale">Geological time</a> <ul><li><a href="/wiki/Age_(geology)" class="mw-redirect" title="Age (geology)">age</a></li> <li><a href="/wiki/Chronozone" title="Chronozone">chron</a></li> <li><a href="/wiki/Eon_(geology)" class="mw-redirect" title="Eon (geology)">eon</a></li> <li><a href="/wiki/Epoch_(geology)" class="mw-redirect" title="Epoch (geology)">epoch</a></li> <li><a href="/wiki/Era_(geology)" class="mw-redirect" title="Era (geology)">era</a></li> <li><a href="/wiki/Geological_period" class="mw-redirect" title="Geological period">period</a></li></ul></li> <li><a href="/wiki/Geochronology" title="Geochronology">Geochronology</a></li> <li><a href="/wiki/Geological_history_of_Earth" title="Geological history of Earth">Geological history of Earth</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:6.5em;font-weight:normal; text-align:center;"><a href="/wiki/Time_in_physics" title="Time in physics">Physics</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Absolute_space_and_time" title="Absolute space and time">Absolute space and time</a></li> <li><a href="/wiki/Arrow_of_time" title="Arrow of time">Arrow of time</a></li> <li><a href="/wiki/Chronon" title="Chronon">Chronon</a></li> <li><a href="/wiki/Coordinate_time" title="Coordinate time">Coordinate time</a></li> <li><a href="/wiki/Instant" title="Instant">Instant</a></li> <li><a href="/wiki/Proper_time" title="Proper time">Proper time</a></li> <li><a href="/wiki/Spacetime" title="Spacetime">Spacetime</a></li> <li><a href="/wiki/Theory_of_relativity" title="Theory of relativity">Theory of relativity</a></li> <li><a href="/wiki/Time_domain" title="Time domain">Time domain</a></li> <li><a href="/wiki/Time_translation_symmetry" class="mw-redirect" title="Time translation symmetry">Time translation symmetry</a></li> <li><a href="/wiki/T-symmetry" title="T-symmetry">Time reversal symmetry</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:6.5em;font-weight:normal; text-align:center;">Other fields</th><td class="navbox-list-with-group navbox-list navbox-even" style="padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Chronological_dating" title="Chronological dating">Chronological dating</a></li> <li><a href="/wiki/Chronobiology" title="Chronobiology">Chronobiology</a> <ul><li><a href="/wiki/Circadian_rhythm" title="Circadian rhythm">Circadian rhythms</a></li></ul></li> <li><a href="/wiki/Chemical_clock" title="Chemical clock">Clock reaction</a></li> <li><a href="/wiki/Glottochronology" title="Glottochronology">Glottochronology</a></li> <li><a href="/wiki/Time_geography" title="Time geography">Time geography</a></li></ul> </div></td></tr></tbody></table><div></div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Related</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Leap_year" title="Leap year">Leap year</a></li> <li><a href="/wiki/Memory" title="Memory">Memory</a></li> <li><a href="/wiki/Moment_(unit)" title="Moment (unit)">Moment</a></li> <li><a href="/wiki/Space" title="Space">Space</a></li> <li><a href="/wiki/System_time" title="System time">System time</a></li> <li><i><a href="/wiki/Tempus_fugit" title="Tempus fugit">Tempus fugit</a></i></li> <li><a href="/wiki/Time_capsule" title="Time capsule">Time capsule</a></li> <li><a href="/wiki/Time_immemorial" title="Time immemorial">Time immemorial</a></li> <li><a href="/wiki/Time_travel" title="Time travel">Time travel</a></li></ul> </div></td></tr><tr><td class="navbox-abovebelow" colspan="2"><div> <ul><li><span class="noviewer" typeof="mw:File"><span title="Category"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/9/96/Symbol_category_class.svg/16px-Symbol_category_class.svg.png" decoding="async" width="16" height="16" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/9/96/Symbol_category_class.svg/23px-Symbol_category_class.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/9/96/Symbol_category_class.svg/31px-Symbol_category_class.svg.png 2x" data-file-width="180" data-file-height="185" /></span></span> <a href="/wiki/Category:Time" title="Category:Time">Category</a></li> <li><span class="noviewer" typeof="mw:File"><span title="Commons page"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/12px-Commons-logo.svg.png" decoding="async" width="12" height="16" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/18px-Commons-logo.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/24px-Commons-logo.svg.png 2x" data-file-width="1024" data-file-height="1376" /></span></span> <a href="https://commons.wikimedia.org/wiki/Category:Time" class="extiw" title="commons:Category:Time">Commons</a></li></ul> </div></td></tr></tbody></table></div> <div class="navbox-styles"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1129693374"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236075235"></div><div role="navigation" class="navbox" aria-labelledby="Time_measurement_and_standards" style="padding:3px"><table class="nowraplinks hlist mw-collapsible autocollapse navbox-inner" style="border-spacing:0;background:transparent;color:inherit"><tbody><tr><th scope="col" class="navbox-title" colspan="3"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1129693374"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1239400231"><div class="navbar plainlinks hlist navbar-mini"><ul><li class="nv-view"><a href="/wiki/Template:Time_measurement_and_standards" title="Template:Time measurement and standards"><abbr title="View this template">v</abbr></a></li><li class="nv-talk"><a href="/wiki/Template_talk:Time_measurement_and_standards" title="Template talk:Time measurement and standards"><abbr title="Discuss this template">t</abbr></a></li><li class="nv-edit"><a href="/wiki/Special:EditPage/Template:Time_measurement_and_standards" title="Special:EditPage/Template:Time measurement and standards"><abbr title="Edit this template">e</abbr></a></li></ul></div><div id="Time_measurement_and_standards" style="font-size:114%;margin:0 4em"><a href="/wiki/Time" title="Time">Time measurement</a> and <a href="/wiki/Time_standard" title="Time standard">standards</a></div></th></tr><tr><td class="navbox-abovebelow" colspan="3"><div> <ul><li><a href="/wiki/Chronometry" title="Chronometry">Chronometry</a></li> <li><a href="/wiki/Orders_of_magnitude_(time)" title="Orders of magnitude (time)">Orders of magnitude</a></li> <li><a href="/wiki/Time_metrology" class="mw-redirect" title="Time metrology">Metrology</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">International standards</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Coordinated_Universal_Time" title="Coordinated Universal Time">Coordinated Universal Time</a> <ul><li><a href="/wiki/UTC_offset" title="UTC offset">offset</a></li></ul></li> <li><a href="/wiki/Universal_Time" title="Universal Time">UT</a></li> <li><a href="/wiki/%CE%94T_(timekeeping)" title="ΔT (timekeeping)">ΔT</a></li> <li><a href="/wiki/DUT1" title="DUT1">DUT1</a></li> <li><a href="/wiki/International_Earth_Rotation_and_Reference_Systems_Service" title="International Earth Rotation and Reference Systems Service">International Earth Rotation and Reference Systems Service</a></li> <li><a href="/wiki/ISO_31-1" title="ISO 31-1">ISO 31-1</a></li> <li><a href="/wiki/ISO_8601" title="ISO 8601">ISO 8601</a></li> <li><a class="mw-selflink selflink">International Atomic Time</a></li> <li><a href="/wiki/12-hour_clock" title="12-hour clock">12-hour clock</a></li> <li><a href="/wiki/24-hour_clock" title="24-hour clock">24-hour clock</a></li> <li><a href="/wiki/Barycentric_Coordinate_Time" title="Barycentric Coordinate Time">Barycentric Coordinate Time</a></li> <li><a href="/wiki/Barycentric_Dynamical_Time" title="Barycentric Dynamical Time">Barycentric Dynamical Time</a></li> <li><a href="/wiki/Civil_time" title="Civil time">Civil time</a></li> <li><a href="/wiki/Daylight_saving_time" title="Daylight saving time">Daylight saving time</a></li> <li><a href="/wiki/Geocentric_Coordinate_Time" title="Geocentric Coordinate Time">Geocentric Coordinate Time</a></li> <li><a href="/wiki/International_Date_Line" title="International Date Line">International Date Line</a></li> <li><a href="/wiki/IERS_Reference_Meridian" title="IERS Reference Meridian">IERS Reference Meridian</a></li> <li><a href="/wiki/Leap_second" title="Leap second">Leap second</a></li> <li><a href="/wiki/Solar_time" title="Solar time">Solar time</a></li> <li><a href="/wiki/Terrestrial_Time" title="Terrestrial Time">Terrestrial Time</a></li> <li><a href="/wiki/Time_zone" title="Time zone">Time zone</a></li> <li><a href="/wiki/180th_meridian" title="180th meridian">180th meridian</a></li></ul> </div></td><td class="noviewer navbox-image" rowspan="9" style="width:1px;padding:0 0 0 2px"><div><span typeof="mw:File"><a href="/wiki/Hourglass" title="Hourglass"><img alt="template illustration" src="//upload.wikimedia.org/wikipedia/commons/thumb/7/70/Marine_sandglass_MMM.jpg/75px-Marine_sandglass_MMM.jpg" decoding="async" width="75" height="175" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/7/70/Marine_sandglass_MMM.jpg/113px-Marine_sandglass_MMM.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/7/70/Marine_sandglass_MMM.jpg/150px-Marine_sandglass_MMM.jpg 2x" data-file-width="1637" data-file-height="3819" /></a></span><br /><span typeof="mw:File"><a href="/wiki/Time_zone" title="Time zone"><img alt="template illustration" src="//upload.wikimedia.org/wikipedia/commons/thumb/6/63/Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png/75px-Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png" decoding="async" width="75" height="152" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/6/63/Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png/113px-Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/6/63/Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png/150px-Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png 2x" data-file-width="496" data-file-height="1007" /></a></span></div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Obsolete standards</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Ephemeris_time" title="Ephemeris time">Ephemeris time</a></li> <li><a href="/wiki/Greenwich_Mean_Time" title="Greenwich Mean Time">Greenwich Mean Time</a></li> <li><a href="/wiki/Prime_meridian" title="Prime meridian">Prime meridian</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Time_in_physics" title="Time in physics">Time in physics</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Absolute_space_and_time" title="Absolute space and time">Absolute space and time</a></li> <li><a href="/wiki/Spacetime" title="Spacetime">Spacetime</a></li> <li><a href="/wiki/Chronon" title="Chronon">Chronon</a></li> <li><a href="/wiki/Continuous_signal" class="mw-redirect" title="Continuous signal">Continuous signal</a></li> <li><a href="/wiki/Coordinate_time" title="Coordinate time">Coordinate time</a></li> <li><a href="/wiki/Cosmological_decade" title="Cosmological decade">Cosmological decade</a></li> <li><a href="/wiki/Discrete_time_and_continuous_time" title="Discrete time and continuous time">Discrete time and continuous time</a></li> <li><a href="/wiki/Proper_time" title="Proper time">Proper time</a></li> <li><a href="/wiki/Theory_of_relativity" title="Theory of relativity">Theory of relativity</a></li> <li><a href="/wiki/Time_dilation" title="Time dilation">Time dilation</a></li> <li><a href="/wiki/Gravitational_time_dilation" title="Gravitational time dilation">Gravitational time dilation</a></li> <li><a href="/wiki/Time_domain" title="Time domain">Time domain</a></li> <li><a href="/wiki/Time-translation_symmetry" title="Time-translation symmetry">Time-translation symmetry</a></li> <li><a href="/wiki/T-symmetry" title="T-symmetry">T-symmetry</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Horology" class="mw-redirect" title="Horology">Horology</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Clock" title="Clock">Clock</a></li> <li><a href="/wiki/Astrarium" title="Astrarium">Astrarium</a></li> <li><a href="/wiki/Atomic_clock" title="Atomic clock">Atomic clock</a></li> <li><a href="/wiki/Complication_(horology)" title="Complication (horology)">Complication</a></li> <li><a href="/wiki/History_of_timekeeping_devices" title="History of timekeeping devices">History of timekeeping devices</a></li> <li><a href="/wiki/Hourglass" title="Hourglass">Hourglass</a></li> <li><a href="/wiki/Marine_chronometer" title="Marine chronometer">Marine chronometer</a></li> <li><a href="/wiki/Marine_sandglass" title="Marine sandglass">Marine sandglass</a></li> <li><a href="/wiki/Radio_clock" title="Radio clock">Radio clock</a></li> <li><a href="/wiki/Watch" title="Watch">Watch</a> <ul><li><a href="/wiki/Stopwatch" title="Stopwatch">stopwatch</a></li></ul></li> <li><a href="/wiki/Water_clock" title="Water clock">Water clock</a></li> <li><a href="/wiki/Sundial" title="Sundial">Sundial</a></li> <li><a href="/wiki/Dialing_scales" title="Dialing scales">Dialing scales</a></li> <li><a href="/wiki/Equation_of_time" title="Equation of time">Equation of time</a></li> <li><a href="/wiki/History_of_sundials" title="History of sundials">History of sundials</a></li> <li><a href="/wiki/Schema_for_horizontal_dials" title="Schema for horizontal dials">Sundial markup schema</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Calendar" title="Calendar">Calendar</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Gregorian_calendar" title="Gregorian calendar">Gregorian</a></li> <li><a href="/wiki/Hebrew_calendar" title="Hebrew calendar">Hebrew</a></li> <li><a href="/wiki/Hindu_calendar" title="Hindu calendar">Hindu</a></li> <li><a href="/wiki/Holocene_calendar" title="Holocene calendar">Holocene</a></li> <li><a href="/wiki/Islamic_calendar" title="Islamic calendar">Islamic</a> (lunar Hijri)</li> <li><a href="/wiki/Julian_calendar" title="Julian calendar">Julian</a></li> <li><a href="/wiki/Solar_Hijri_calendar" title="Solar Hijri calendar">Solar Hijri</a></li> <li><a href="/wiki/Astronomical_year_numbering" title="Astronomical year numbering">Astronomical</a></li> <li><a href="/wiki/Dominical_letter" title="Dominical letter">Dominical letter</a></li> <li><a href="/wiki/Epact" title="Epact">Epact</a></li> <li><a href="/wiki/Equinox" title="Equinox">Equinox</a></li> <li><a href="/wiki/Intercalation_(timekeeping)" title="Intercalation (timekeeping)">Intercalation</a></li> <li><a href="/wiki/Julian_day" title="Julian day">Julian day</a></li> <li><a href="/wiki/Leap_year" title="Leap year">Leap year</a></li> <li><a href="/wiki/Lunar_calendar" title="Lunar calendar">Lunar</a></li> <li><a href="/wiki/Lunisolar_calendar" title="Lunisolar calendar">Lunisolar</a></li> <li><a href="/wiki/Solar_calendar" title="Solar calendar">Solar</a></li> <li><a href="/wiki/Solstice" title="Solstice">Solstice</a></li> <li><a href="/wiki/Tropical_year" title="Tropical year">Tropical year</a></li> <li><a href="/wiki/Determination_of_the_day_of_the_week" title="Determination of the day of the week">Weekday determination</a></li> <li><a href="/wiki/Names_of_the_days_of_the_week" title="Names of the days of the week">Weekday names</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Archaeology and geology</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Chronological_dating" title="Chronological dating">Chronological dating</a></li> <li><a href="/wiki/Geologic_time_scale" title="Geologic time scale">Geologic time scale</a></li> <li><a href="/wiki/International_Commission_on_Stratigraphy" title="International Commission on Stratigraphy">International Commission on Stratigraphy</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Astronomical_chronology" title="Astronomical chronology">Astronomical chronology</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Galactic_year" title="Galactic year">Galactic year</a></li> <li><a href="/wiki/Nuclear_timescale" title="Nuclear timescale">Nuclear timescale</a></li> <li><a href="/wiki/Precession" title="Precession">Precession</a></li> <li><a href="/wiki/Sidereal_time" title="Sidereal time">Sidereal time</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Other <a href="/wiki/Unit_of_time" title="Unit of time">units of time</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Instant" title="Instant">Instant</a></li> <li><a href="/wiki/Flick_(time)" title="Flick (time)">Flick</a></li> <li><a href="/wiki/Shake_(unit)" title="Shake (unit)">Shake</a></li> <li><a href="/wiki/Jiffy_(time)" title="Jiffy (time)">Jiffy</a></li> <li><a href="/wiki/Second" title="Second">Second</a></li> <li><a href="/wiki/Minute" title="Minute">Minute</a></li> <li><a href="/wiki/Moment_(unit)" title="Moment (unit)">Moment</a></li> <li><a href="/wiki/Hour" title="Hour">Hour</a></li> <li><a href="/wiki/Day" title="Day">Day</a></li> <li><a href="/wiki/Week" title="Week">Week</a></li> <li><a href="/wiki/Fortnight" title="Fortnight">Fortnight</a></li> <li><a href="/wiki/Month" title="Month">Month</a></li> <li><a href="/wiki/Year" title="Year">Year</a></li> <li><a href="/wiki/Olympiad" title="Olympiad">Olympiad</a></li> <li><a href="/wiki/Lustrum" title="Lustrum">Lustrum</a></li> <li><a href="/wiki/Decade" title="Decade">Decade</a></li> <li><a href="/wiki/Century" title="Century">Century</a></li> <li><a href="/wiki/Saeculum" title="Saeculum">Saeculum</a></li> <li><a href="/wiki/Millennium" title="Millennium">Millennium</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Related topics</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Chronology" title="Chronology">Chronology</a></li> <li><a href="/wiki/Duration_(philosophy)" title="Duration (philosophy)">Duration</a> <ul><li><a href="/wiki/Duration_(music)" title="Duration (music)">music</a></li></ul></li> <li><a href="/wiki/Mental_chronometry" title="Mental chronometry">Mental chronometry</a></li> <li><a href="/wiki/Decimal_time" title="Decimal time">Decimal time</a></li> <li><a href="/wiki/Metric_time" title="Metric time">Metric time</a></li> <li><a href="/wiki/System_time" title="System time">System time</a></li> <li><a href="/wiki/Time_metrology" class="mw-redirect" title="Time metrology">Time metrology</a></li> <li><a href="/wiki/Time_value_of_money" title="Time value of money">Time value of money</a></li> <li><a href="/wiki/Timekeeper" title="Timekeeper">Timekeeper</a></li></ul> </div></td></tr></tbody></table></div> <div class="navbox-styles"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1129693374"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236075235"></div><div role="navigation" class="navbox authority-control" aria-label="Navbox" style="padding:3px"><table class="nowraplinks hlist navbox-inner" style="border-spacing:0;background:transparent;color:inherit"><tbody><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Help:Authority_control" title="Help:Authority control">Authority control databases</a> <span 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