Coordinated Universal Time
UTC has been widely embraced by most countries and is the effective successor to Greenwich Mean Time (GMT) in everyday usage and common applications.
[a] In specialized domains such as scientific research, navigation, and timekeeping, other standards such as UT1 and International Atomic Time (TAI) are also used alongside UTC.
The General Conference on Weights and Measures adopted a resolution to alter UTC with a new system that would eliminate leap seconds by 2035.
The name "Coordinated Universal Time (UTC)" was approved by a resolution of IAU Commissions 4 and 31 at the 13th General Assembly in 1967 (Trans.
Decisions to introduce a leap second are announced at least six months in advance in "Bulletin C" produced by the International Earth Rotation and Reference Systems Service.
It accounts for the grand total of the extra length (about 2 milliseconds each) of all the mean solar days since the previous leap 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.
In order to maintain a close approximation to UT1, UTC occasionally has discontinuities where it changes from one linear function of TAI to another.
[17] The time of events may be provisionally recorded against one of these approximations; later corrections may be applied using the International Bureau of Weights and Measures (BIPM) monthly publication of tables of differences between canonical TAI/UTC and TAI(k)/UTC(k) as estimated in real-time by participating laboratories.
Because of time dilation, a standard clock not on the geoid, or in rapid motion, will not maintain synchronicity with UTC.
Time zones are usually defined as differing from UTC by an integer number of hours,[19] 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 quarter-hours from UT1 or UTC.
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 Accra in Ghana or Reykjavík in Iceland as they are always on UTC and do not currently use daylight saving time (which Greenwich and London do, and so could be a source of error).
[24] In 1928, the term Universal Time (UT) was introduced by the International Astronomical Union to refer to GMT, with the day starting at midnight.
When the divergence grew significantly, the signal was phase shifted (stepped) by 20 ms to bring it back into agreement with UT.
This publication allowed a value to be chosen for the length of the atomic second that would accord with the celestial laws of motion.
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.
In 1968, Louis Essen, the inventor of the caesium atomic clock, and G. M. R. Winkler both independently proposed that steps should be of 1 second only.
This CCIR Recommendation 460 "stated that (a) carrier frequencies and time intervals should be maintained constant and should correspond to the definition of the SI second; (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.
[37][38] The current version of UTC is defined by International Telecommunication Union Recommendation (ITU-R TF.460-6), Standard-frequency and time-signal emissions,[39] and is based on International Atomic Time (TAI) with leap seconds added at irregular intervals to compensate for the accumulated difference between TAI and time measured by Earth's rotation.
[42] A study published in March 2024 in Nature 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.
[43] Earth's rotational speed is very slowly decreasing because of tidal deceleration; this increases the length of the mean solar day.
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.
[46] The actual rotational period varies on unpredictable factors such as tectonic motion and has to be observed, rather than computed.
While the rate due to tidal friction alone is about 2.3 ms/cy, the uplift of Canada and Scandinavia by several metres since the last ice age has temporarily reduced this to 1.7 ms/cy over the last 2,700 years.
In April 2001, Rob Seaman of the National Optical Astronomy Observatory proposed that leap seconds be allowed to be added monthly rather than twice yearly.
[52] Thus, the time system will lose its fixed connection to the geographic coordinates based on the IERS meridian.
[53] ITU-R 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),[54] but consideration of the proposal was postponed by the ITU until the World Radio Conference in 2015.
[55] This conference, in turn, considered the question,[56] but no permanent decision was reached; it only chose to engage in further study with the goal of reconsideration in 2023.
[58] 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 Resolution 4 of the 27th CGPM (2022) which decides that the maximum value for the difference (UT1-UTC) will be increased in, or before, 2035.
