Coordinated Universal Time

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position on the graph corresponds to the accumulation of this difference over time, and the vertical segments correspond to leap seconds introduced to match this accumulated difference. Leap seconds are timed to keep DUT1 within the vertical range depicted by the adjacent graph. The frequency of leap seconds therefore corresponds to the slope of the diagonal graph segments, and thus to the excess LOD. Time periods when the slope reverses direction (slopes upwards, not the vertical segments) are times when the excess LOD is negative, that is, when the LOD is below 86,400 s.

667:. The ephemeris second is a unit in the system of time that, when used as the independent variable in the laws of motion that govern the movement of the planets and moons in the solar system, enables the laws of motion to accurately predict the observed positions of solar system bodies. Within the limits of observable accuracy, ephemeris seconds are of constant length, as are atomic seconds. This publication allowed a value to be chosen for the length of the atomic second that would accord with the celestial laws of motion. 694:

different lengths, namely the UTC second and the SI second used in TAI, was a bad idea. It was thought better for time signals to maintain a consistent frequency, and that this frequency should match the SI second. Thus it would be necessary to rely on time steps alone to maintain the approximation of UT. This was tried experimentally in a service known as "Stepped Atomic Time" (SAT), which ticked at the same rate as TAI and used jumps of 0.2 seconds to stay synchronised with UT2.

4477: 4258: 68: 778: 4268: 966: 295:, it may be 61 or 59 instead. Thus, in the UTC time scale, the second and all smaller time units (millisecond, microsecond, etc.) are of constant duration, but the minute and all larger time units (hour, day, week, etc.) are of variable duration. Decisions to introduce a leap second are announced at least six months in advance in "Bulletin C" produced by the 905:

beginning around June 2019 in which instead of slowing down (with leap seconds to keep the difference between UT1 and UTC less than 0.9 seconds) the Earth's rotation has sped up, causing this difference to increase. If the trend continues, a negative leap second may be required, which has not been used before. This may not be needed until 2025.

242:(NTP), designed to synchronise the clocks of computers over the Internet, transmits time information from the UTC system. If only milliseconds precision is needed, clients can obtain the current UTC from a number of official internet UTC servers. For sub-microsecond precision, clients can obtain the time from satellite signals. 932:

length. This would be a change in civil timekeeping, and would have a slow effect at first, but becoming drastic over several centuries. UTC (and TAI) would be more and more ahead of UT; it would coincide with local mean time along a meridian drifting eastward faster and faster. Thus, the time system

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second was redefined in terms of the frequency supplied by a caesium atomic clock. The length of second so defined was practically equal to the second of ephemeris time. This was the frequency that had been provisionally used in TAI since 1958. It was soon decided that having two types of second with

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began coordinating the UTC process internationally (but the name Coordinated Universal Time was not formally adopted by the International Astronomical Union until 1967). From then on, there were time steps every few months, and frequency changes at the end of each year. The jumps increased in size to

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without consulting a table showing how many leap seconds occurred during that interval. By extension, it is not possible to compute the precise duration of a time interval that ends in the future and may encompass an unknown number of leap seconds (for example, the number of TAI seconds between "now"

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is slightly longer than 86,400 SI seconds so occasionally the last minute of a UTC day is adjusted to have 61 seconds. The extra second is called a leap second. It accounts for the grand total of the extra length (about 2 milliseconds each) of all the mean solar days since the previous leap

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Study Group 7 and Working Party 7A were unable to reach consensus on whether to advance the proposal to the 2012 Radiocommunications Assembly; the chairman of Study Group 7 elected to advance the question to the 2012 Radiocommunications Assembly (20 January 2012), but consideration of

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would slowly get further and further out of sync with civil time. The leap seconds will be eliminated by 2035. The resolution does not break the connection between UTC and UT1, but increases the maximum allowable difference. The details of what the maximum difference will be and how corrections will

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Just as adding a leap day every four years does not mean the year is getting longer by one day every four years, the insertion of a leap second every 800 days does not indicate that the mean solar day is getting longer by a second every 800 days. It will take about 50,000 years for a mean solar day

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of a mean solar day in the mid‑19th century. In earlier centuries, the mean solar day was shorter than 86,400 SI seconds, and in more recent centuries it is longer than 86,400 seconds. Near the end of the 20th century, the length of the mean solar day (also known simply as "length of day" or "LOD")

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Some time in the 22nd century, two leap seconds will be required every year. The current practice of only allowing leap seconds in June and December will be insufficient to maintain a difference of less than 1 second, and it might be decided to introduce leap seconds in March and September. In the

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exactly, and a whole number of seconds thereafter. At the same time, the tick rate of UTC was changed to exactly match TAI. UTC also started to track UT1 rather than UT2. Some time signals started to broadcast the DUT1 correction (UT1 − UTC) for applications requiring a closer approximation of UT1

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where it changes from one linear function of TAI to another. These discontinuities take the form of leap seconds implemented by a UTC day of irregular length. Discontinuities in UTC occurred only at the end of June or December. However, there is provision for them to happen at the end of March and

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The excess of the LOD over the nominal 86,400 s accumulates over time, causing the UTC day, initially synchronised with the mean sun, to become desynchronised and run ahead of it. Near the end of the 20th century, with the LOD at 1.3 ms above the nominal value, UTC ran faster than UT by

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In a controversial decision, the frequency of the signals was initially set to match the rate of UT, but then kept at the same frequency by the use of atomic clocks and deliberately allowed to drift away from UT. When the divergence grew significantly, the signal was phase shifted (stepped) by 20

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above, the excess of LOD above the nominal 86,400 s corresponds to the downward slope of the graph between vertical segments. (The slope became shallower in the 1980s, 2000s and late 2010s to 2020s because of slight accelerations of Earth's rotation temporarily shortening the day.) Vertical

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adopted the names Coordinated Universal Time and Temps Universel Coordonné for the English and French names with the acronym UTC to be used in both languages. The name "Coordinated Universal Time (UTC)" was approved by a resolution of IAU Commissions 4 and 31 at the 13th General Assembly in 1967

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in 1972. Recent years have seen significant developments in the realm of UTC, particularly in discussions about eliminating leap seconds from the timekeeping system because leap seconds occasionally disrupt timekeeping systems worldwide. The General Conference on Weights and Measures adopted a

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of LOD is approximately +1.7 ms per century. At the end of the 21st century, LOD will be roughly 86,400.004 s, requiring leap seconds every 250 days. Over several centuries, the frequency of leap seconds will become problematic. A change in the trend of the UT1 – UTC values was seen

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Time zones are usually defined as differing from UTC by an integer number of hours, although the laws of each jurisdiction would have to be consulted if sub-second accuracy was required. Several jurisdictions have established time zones that differ by an odd integer number of half-hours or

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The first leap second occurred on 30 June 1972. Since then, leap seconds have occurred on average about once every 19 months, always on 30 June or 31 December. As of July 2022, there have been 27 leap seconds in total, all positive, putting UTC 37 seconds behind TAI.

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in a new UTC in 1970 and implemented in 1972, along with the idea of maintaining the UTC second equal to the TAI second. This CCIR Recommendation 460 "stated that (a) carrier frequencies and time intervals should be maintained constant and should correspond to the definition of the

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The coordination of time and frequency transmissions around the world began on 1 January 1960. UTC was first officially adopted as a standard in 1963 and "UTC" became the official abbreviation of Coordinated Universal Time in 1967. The current version of UTC is defined by the

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or civil time may change if a time zone jurisdiction observes daylight saving time (summer time). For example, local time on the east coast of the United States is five hours behind UTC during winter, but four hours behind while daylight saving is observed there.

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be implemented is left for future discussions. This will result in a shift of the sun's movements relative to civil time, with the difference increasing quadratically with time (i.e., proportional to elapsed centuries squared). This is analogous to the shift of

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second. The last minute of a UTC day is permitted to contain 59 seconds to cover the remote possibility of the Earth rotating faster, but that has not yet been necessary. The irregular day lengths mean fractional Julian days do not work properly with UTC.

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the proposal was postponed by the ITU until the World Radio Conference in 2015. This conference, in turn, considered the question, but no permanent decision was reached; it only chose to engage in further study with the goal of reconsideration in 2023.

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Resolution no. 3 by Commissions 4 (Ephemerides/Ephémérides) and 31 (Time/L'Heure) (near the end of the document) "recommend that the following notations be used in all languages", UT0(i), UT1(i), UT2(i), UTC, UTC(i), UT, where (i) is institution

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As an intermediate step at the end of 1971, there was a final irregular jump of exactly 0.107758 TAI seconds, making the total of all the small time steps and frequency shifts in UTC or TAI during 1958–1971 exactly ten seconds, so that

1126: 701:, the inventor of the caesium atomic clock, and G. M. R. Winkler both independently proposed that steps should be of 1 second only. to simplify future adjustments. This system was eventually approved as 768:

concluded that accelerated melting of ice in Greenland and Antarctica due to climate change has decreased Earth's rotational velocity, affecting UTC adjustments and causing problems for computer networks that rely on UTC.

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As with TAI, UTC is only known with the highest precision in retrospect. Users who require an approximation in real time must obtain it from a time laboratory, which disseminates an approximation using techniques such as

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and 2099-12-31 23:59:59). Therefore, many scientific applications that require precise measurement of long (multi-year) intervals use TAI instead. TAI is also commonly used by systems that cannot handle leap seconds.

710:; (b) step adjustments, when necessary, should be exactly 1 s to maintain approximate agreement with Universal Time (UT); and (c) standard signals should contain information on the difference between UTC and UT." 2722:

The Earth's Rotation and Reference Frames for Geodesy and Geophysics: Proceedings of the 128th Symposium of the International Astronomical Union, held in Coolfont, West Virginia, U.S.A., 20–24 October 1986

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The pips are no longer broadcast from Greenwich, but from the National Physical Laboratory in Teddington, Surrey, which uses Coordinated Universal Time (UTC) – the successor of GMT – for its reading.

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1.3 ms per day, getting a second ahead roughly every 800 days. Thus, leap seconds were inserted at approximately this interval, retarding UTC to keep it synchronised in the long term. The actual

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is "Zulu", UTC is sometimes known as "Zulu time". This is especially true in aviation, where "Zulu" is the universal standard. This ensures that all pilots, regardless of location, are using the same

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has temporarily reduced this to 1.7 ms/cy over the last 2,700 years. The correct reason for leap seconds, then, is not the current difference between actual and nominal LOD, but rather the

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In 2022 a resolution was adopted by the General Conference on Weights and Measures to redefine UTC and abolish leap seconds, but keep the civil second constant and equal to the SI second, so that

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coordinated their radio broadcasts so that time steps and frequency changes were coordinated, and the resulting time scale was informally referred to as "Coordinated Universal Time".

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September as well as a second preference. The International Earth Rotation and Reference Systems Service (IERS) tracks and publishes the difference between UTC and Universal Time,

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is an abbreviation for the time laboratory. The time of events may be provisionally recorded against one of these approximations; later corrections may be applied using the

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ITU World Radiocommunication Conference 2023 (WRC-23), which was held in Dubai (United Arab Emirates) from 20 November to 15 December 2023 formally recognized the

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time signals, named for the shortwave radio station that broadcasts them. In 1960, the U.S. Naval Observatory, the Royal Greenwich Observatory, and the

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can also be used. Each day contains 24 hours and each hour contains 60 minutes. The number of seconds in a minute is usually 60, but with an occasional

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On electronic devices which only allow the time zone to be configured using maps or city names, UTC can be selected indirectly by selecting cities such as

101:(GMT) in everyday usage and common applications. In specialized domains such as scientific research, navigation, and timekeeping, other standards such as 3202: 632:

was invented. This provided a form of timekeeping that was both more stable and more convenient than astronomical observations. In 1956, the U.S.

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to lengthen by one second (at a rate of 2 ms per century). This rate fluctuates within the range of 1.7–2.3 ms/cy. While the rate due to

2554: 2529: 2882: 2819: 2272: 2159: 1774:, p. 230. (Average for period from 1 January 1991 through 1 January 2009. Average varies considerably depending on what period is chosen.) 1663: 1180: 645: 2960: 2774: 4297: 2186: 1355: 826:

was approximately 86,400.0013 s. For this reason, UT is now "slower" than TAI by the difference (or "excess" LOD) of 1.3 ms/day.

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The coordination of time and frequency transmissions around the world began on 1 January 1960. UTC was first officially adopted in 1963 as

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Finkleman, David; Allen, Steve; Seago, John; Seaman, Rob; Seidelmann, P. Kenneth (2011). "The Future of Time: UTC and the Leap Second".

618:) was introduced by the International Astronomical Union to refer to GMT, with the day starting at midnight. Until the 1950s, broadcast 5073: 2488: 909:

25th century, four leap seconds are projected to be required every year, so the current quarterly options would be insufficient.

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operators often schedule their radio contacts in UTC, because transmissions on some frequencies can be picked up in many time zones.

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Nelson, Robert A.; McCarthy, Dennis D.; Malys, S.; Levine, J.; Guinot, B.; Fliegel, H. F.; Beard, R. L.; Bartholomew, T. R. (2001).

4835: 738:(TAI) with leap seconds added at irregular intervals to compensate for the accumulated difference between TAI and time measured by 522:

as it was the point of origin. The letter also refers to the "zone description" of zero hours, which has been used since 1920 (see

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of the 27th CGPM (2022) which decides that the maximum value for the difference (UT1-UTC) will be increased in, or before, 2035.

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A proposed alternative to the leap second is the leap hour or leap minute, which requires changes only once every few centuries.

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since about 1950. Time zones were identified by successive letters of the alphabet and the Greenwich time zone was marked by a

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from clocks with a known relation to the geoid is used to provide UTC when required, on locations such as those of spacecraft.

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of a second per day; therefore, after about 800 days, it accumulated to 1 second (and a leap second was then added).

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Markowitz, Wm. (1988). "Comparisons of ET (Solar), ET (Lunar), UT and TDT". In Babcock, A. K.; Wilkins, G. A. (eds.).

2635: 4555: 3182: 3056: 2724:. International Astronomical Union Symposia. Vol. 128. Dordrecht: Kluwer Academic Publishers. pp. 413–418. 2446: 1833: 355: 168:, which conforms to the pattern for the abbreviations of the variants of Universal Time (UT0, UT1, UT2, UT1R, etc.). 2071: 161: 3210: 86:

globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for

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As the Earth's rotation continues to slow, positive leap seconds will be required more frequently. The long-term

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always remains exactly 19 seconds behind TAI (neither system is affected by the leap seconds introduced in UTC).

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Stephenson, F. R.; Morrison, L. V. (1995). "Long-term fluctuations in the Earth's rotation: 700 BC to AD 1990".

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and can now be seen to have a relationship with the mean solar day observed between 1750 and 1892, analysed by

410:, a standard clock not on the geoid, or in rapid motion, will not maintain synchronicity with UTC. Therefore, 299:. The leap seconds cannot be predicted far in advance due to the unpredictable rate of the rotation of Earth. 5549: 4809: 303: 3779: 3295: 1938: 862:

of this difference over a period of time: Near the end of the 20th century, this difference was about

633: 2912: 4764: 4744: 4690: 4575: 4411: 4070: 2898: 2842: 2815: 2793: 2747: 937:. The difference between UTC and UT would reach 0.5 hours after the year 2600 and 6.5 hours around 4600. 539: 465: 262: 2686: 640:

started to develop atomic frequency time scales; by 1959, these time scales were used in generating the

4431: 4416: 4333: 3422: 2158:, National Bureau of Standards, National Institute of Standards and Technology since 1988, p. 32, 1019: 1259: 5059: 4585: 4396: 4133: 3982: 3862: 3412: 3325: 2908: 2525: 735: 682: 400: 331: 106: 30: 2233: 2150: 3832: 449: 197: 3274:

International Earth Rotation Service; list of differences between TAI and UTC from 1961 to present

1960: 5585: 5260: 5186: 4845: 4623: 4525: 4441: 4436: 4088: 3739: 2921: 2751: 1001: 934: 319: 2178: 3442: 2392: 979: 928:

relative to the yearly calendar that results from the calendar year not precisely matching the

637: 527: 364: 3273: 3155: 94:. UTC facilitates international communication, navigation, scientific research, and commerce. 4998: 4271: 3903: 3848: 3802: 3787: 3432: 2513: 2292: 985: 239: 2481: 483:

can be determined by adding or subtracting the number of hours and minutes specified by the

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Scientia Canadensis: Canadian Journal of the History of Science, Technology and Medicine

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to bring it back into agreement with UT. Twenty-nine such steps were used before 1960.

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seconds with exactly 60 seconds in each minute. UTC is within about one second of

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and maps all use UTC to avoid confusion about time zones and daylight saving time. The

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wanting to use the same abbreviation in all languages. The compromise that emerged was

2413: 2125: 391:(BIPM) monthly publication of tables of differences between canonical TAI/UTC and TAI( 4858: 4723: 4540: 4058: 3937: 3837: 3690: 3593: 3497: 3095: 3052: 3004: 2956: 2858: 2801: 2733: 2708: 2442: 2380: 2372: 2298: 2294:

It's About Time: From Calendars and Clocks to Moon Cycles and Light Years - A History

2268: 1730: 1702: 1387: 454:"Zulu time" redirects here. For the album by Caspar Brötzmann and Page Hamilton, see 258: 144:

resolution to alter UTC with a new system that would eliminate leap seconds by 2035.

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Definition of Coordinated Universal Time in German law – ZeitG §1 (3)

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proposed that leap seconds be allowed to be added monthly rather than twice yearly.

742:. Leap seconds are inserted as necessary to keep UTC within 0.9 seconds of the 678:, and "UTC" became the official abbreviation of Coordinated Universal Time in 1967. 5249: 5051: 5003: 4749: 4728: 4680: 4643: 4456: 4176: 4164: 3992: 3960: 3792: 3661: 3577: 3567: 3482: 3282: 3083: 3000: 2948: 2944: 2766: 2714: 2658: 2590: 2368: 2339: 2256: 2215: 2038: 1722: 1148: 1025: 971: 790: 764: 660: 330:

Since 1972, UTC may be calculated by subtracting the accumulated leap seconds from

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There was also dissatisfaction with the frequent jumps in UTC (and SAT). In 1968,

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UTC has been widely embraced by most countries and is the effective successor to

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0.1 seconds. This UTC was intended to permit a very close approximation to UT2.

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between UT1 and UTC (in seconds). Vertical segments correspond to leap seconds.

664: 399:) as estimated in real-time by participating laboratories. (See the article on 351: 323: 315: 235: 121: 3104:"15 U.S. Code § 261 – Zones for standard time; interstate or foreign commerce" 2662: 1350: 1119:"Resolutions of the General Conference on Weights and Measures (27th Meeting)" 5600: 5242: 5236: 5231: 5226: 5221: 5216: 5211: 5205: 5199: 5194: 5172: 5167: 5161: 5155: 5150: 5145: 5139: 5133: 5128: 5096: 4917: 4868: 4853: 4799: 4570: 4406: 4401: 4317: 4261: 4211: 4159: 4093: 4065: 4053: 4043: 4033: 3915: 3759: 3685: 3634: 3587: 3502: 3462: 3457: 3452: 3385: 2396: 929: 806: 535: 523: 511: 488: 407: 268: 204: 83: 3237: 1939:"ITU World Radiocommunication Conference set for Geneva, 2–27 November 2015" 1247: 1022: – Recognized maintainers of atomic clocks from which UTC is calculated 417:

It is impossible to compute the exact time interval elapsed between two UTC

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will lose its fixed connection to the geographic coordinates based on the

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Guinot, Bernard (August 2011). "Solar time, legal time, time in use".

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Annual Report of the Board of Regents of the Smithsonian Institution

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Since adoption, UTC has been adjusted several times, notably adding

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CHIPS: The Department of the Navy's Information Technology Magazine

1919: 1612: 1498: 1447: 1007: 1004: – International prime meridian used for GPS and other systems 455: 423: 246: 231: 212: 211:, being fourteen hours ahead of UTC. In 1995, the island nation of 2904:

Coordinated Universal Time (UTC) and the Future of the Leap Second

2326: 2096:. ITU-R Special Rapporteur Group Colloquium on the UTC Time Scale. 1028: – Time standard for astronomical observations from the Earth 671: 538:, thus avoiding confusion when flying between time zones. See the 176: 4967: 4957: 4887: 4759: 4670: 4535: 4108: 4098: 4020: 4011: 3996: 3883: 3728: 3651: 3360: 1789: 1552: 1331: 1307: 1271: 920: 626: 562: 3898: 1753: 1741: 1034: – Time standard based on the slowing rotation of the Earth 207:, being twelve hours behind UTC; the easternmost time zone uses 112:

UTC is based on TAI, which is a weighted average of hundreds of

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International Earth Rotation and Reference Systems Service 2011

1253: 925: 574: 280: 2108:. Aircraft Owners and Pilots Association. 2006. Archived from 1777: 1423: 4942: 4754: 4653: 4608: 4216: 3965: 3656: 3604: 2876:"NIST Time and Frequency Radio Stations: WWV, WWVH, and WWVB" 2634:

Markowitz, W.; Hall, R.; Essen, L.; Parry, J. (August 1958).

1013: 940: 554: 550: 347: 343: 1941:(Press release). International Telecommunication Union. 2015 1513: 1486: 152:

The official abbreviation for Coordinated Universal Time is

4947: 4932: 4922: 4376: 4313: 3355: 3341: 3051:(2nd ed.). Mill Valley, CA: University Science Books. 1600: 882: 782: 750:" section for the number of leap seconds inserted to date. 360: 4382: