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Coordinated Universal Time

From Wikipedia, the free encyclopedia.

(Redirected from UTC)

Coordinated Universal Time or UTC, also sometimes referred to as "Zulu time", 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.

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. 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.

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.



General information

"UTC" is not a true abbreviation; it is a variant of Universal Time, abbreviated UT, and has a modifier C (for "coordinated") appended to it just like other variants of UT. It may be regarded as a compromise between the English abbreviation "CUT" and the French abbreviation "TUC" (temps universel coordonné). It is sometimes erroneously expanded into "Universal Time Code".

Universal Time
to Standard (Winter) Local Times
Pacific Standard Time

Pacific Daylight Time

UTC -8

UTC -7

Mountain Standard Time

Mountain Daylight Time

UTC -7

UTC -6

Central Standard Time

Central Daylight Time

UTC -6

UTC -5

Eastern Standard Time

Eastern Daylight Time

UTC -5

UTC -4

Atlantic Standard Time UTC -4
Greenwich Mean Time UTC
Central European Time UTC +1
Eastern European Time UTC +2
Moscow Time UTC +3
Indian Standard Time UTC +5:30
Singapore Standard Time, Australian Western Standard Time, Hong Kong Time
Chinese Standard Time
UTC +8
Japan Standard Time, Korea Standard Time UTC +9
Australian Central Standard Time UTC +9:30
Australian Eastern Standard Time UTC +10
New Zealand Standard Time UTC +12
For more, see time zone.

International standard UTC time can only be determined to the highest precision after the fact, as atomic time is determined by the reconciliation of the observed differences between an ensemble of atomic clocks maintained by a number of national time bureaus. This is done under the auspices of the Bureau International des Poids et Mesures (International Bureau of Weights and Measures). However, local clusters of atomic clocks are sufficient for accuracy to within a few tens of nanoseconds.

UTC is the time system used for many Internet and World Wide Web standards. In particular, the Network Time Protocol is designed as a way of dynamically distributing time over the Internet.

Several classes of software UTC clocks exist.

  • Relating to the calculation of the hour:
    • Drag when the clock shows the UTC hour calculating it from your local computer clock. You can see if a UTC clock is a drag one by changing your local computer clock: if UTC hour varies, it is a drag UTC clock.
    • Autonomous, if it is not a drag clock. This is the best class of UTC clock.
  • Showing the hour:
    • Static: the time does not change from the latest reload.
    • Dynamic: the time changes from minute to minute.

As indicated in the standards, it is convenient to include the UTC date too.

The UT time zone is sometimes denoted by the letter Z since the equivalent nautical time zone (GMT) has been denoted by Z since about 1950, and by a "zone description" of zero hours since 1920. See Time zone#History. Since the NATO phonetic alphabet and radio-amateur word for Z is "Zulu", UT is sometimes known as Zulu time.


  • ITU-R Recommendation TF.460-4: Standard-frequency and time-signal emissions. International Telecommunication Union. (Annex I of this document contains the official definition of UTC.)
  • Dennis D. McCarthy: "Astronomical Time". Proc. IEEE, Vol. 79, No. 7, July 1991, pp. 915-920.
  • Nelson, McCarthy, et al.: "The leap second: its history and possible future" (381 KB PDF file), Metrologia, Vol. 38, pp. 509–529, 2001.
  • David W. Allan, Neil Ashby, Clifford C. Hodge: The Science of Timekeeping. Hewlett Packard Application Note 1289, 1997.

See also

External links

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