Coordinated Universal Time

:For alternate uses of UTC see UTC (disambiguation)

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Coordinated Universal Time or UTC, also sometimes referred to as "Zulu time" or Z, is an atomic realization of Universal Time (UT) or Greenwich Mean Time, the astronomical basis for civil time. Time zones around the world are expressed as positive and negative offsets from UT. UTC differs by an integral number of seconds from International Atomic Time (TAI), as measured by atomic clocks and a fractional number of seconds from UT.

Related Topics:
Universal Time - Greenwich Mean Time - Civil time - Time zone - Second - International Atomic Time

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UTC is a hybrid time scale: the rate of UTC is based on atomic frequency standards but the epoch of UTC is synchronized to remain close to astronomical UT. The Earth's rotation is very slowly decelerating (due to braking action of the tides), hence the mean solar day has increased since TAI was introduced. For this reason, UT is 'slower' than TAI. As of 1 January 1999, TAI is ahead of UTC by 32 seconds. UTC is maintained within 0.9 s of UT1 (UT1 is one of three precise definitions of UT); leap seconds are added (or, theoretically, subtracted) at the end of any UTC month as necessary. To date, all such adjustments -- the first in 1972 -- have been positive and applied on dates June 30 or December 31, where an additive leap second is designated as 23:59:60. The announcement of leap seconds is made by the International Earth Rotation and Reference Systems Service (IERS), based on precise astronomical forecasts of the Earth's rotation. Historically, one leap second has been required every one to two years. However a leap second has not been required since 1998, as the deceleration of the Earth's rotation slowed temporarily in the past seven years. The IERS announced in July 2005 that the next leap second will be on 31 December 2005.

Related Topics:
Epoch - UT - Leap second - 1972 - International Earth Rotation and Reference Systems Service

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For most practical and legal-trade purposes, the fractional difference between UTC and UT (or, GMT) is inconsequentially small, and for this reason UTC is colloquially called GMT sometimes, even if this is not technically correct.

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