Coordinated Universal Time

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UTC redirects here. For other uses see UTC (disambiguation).

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 (GMT), the astronomical basis for civil time. Time zones around the world are expressed as positive and negative offsets from UT.

This document was sent at Friday, 2025-06-Template:CURRENTDAY2 00:18Z Universal Time Coordinated.

1 General information
2 Abbreviation
3 Leap second adjustments
4 Amateur radio
5 References
6 See also
7 External links

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General information

Originally, the local time at the Royal Greenwich Observatory in Greenwich, England was chosen as standard at the 1884 International Meridian Conference, leading to the widespread use of Greenwich Mean Time in order to set local clocks. This location was chosen because by 1884 two-thirds of all charts and maps already used it as their prime meridian.

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Abbreviation

English speakers and French speakers each wanted the intials of their respective languages' terms to be used internationally: CUT for coordinated universal time and TUC for temps universel coordonné. As a compromise, a variation of the English term - with the verbal adjective trailing as in French - supplied the initials of the abbreviation: "universal time, coordinated" became UTC. [1]

"UTC" is not a true acronym; it is a variant of Universal Time, UT, and has a modifier C (for "coordinated") appended to it just like other variants of UT. It is sometimes erroneously expanded into "Universal Time Code".

Image:Timezones.png Image:Europe time zones map en.png

**Local (Winter) Times relative to Universal Time**
Time zone name	UTC
Hawaii-Aleutian Standard Time	UTC-10
Alaska Standard Time	UTC-9
Pacific Standard Time (USA)	UTC-8
Mountain Standard Time (USA)	UTC-7
Central Standard Time (USA)	UTC-6
Eastern Standard Time (USA)	UTC-5
Atlantic Standard Time (USA)	UTC-4
Newfoundland Standard Time	UTC-3:30
Greenwich Mean Time/Western European Time	UTC
Central European Time	[[UTC+1]]
Eastern European Time, South African Standard Time	[[UTC+2]]
Moscow Time	[[UTC+3]]
Pakistan Standard Time	[[UTC+5]]
Indian Standard Time	[[UTC+5:30]]
Singapore Standard Time Hong Kong Time Australian Western Standard 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.

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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, designed to synchronize the clocks of many computers over the Internet (usually to that of a known accurate atomic clock), uses UTC. Because of general relativity a standard clock not on the geoid, or in rapid motion, will not maintain synchronism with UTC. Therefore, telemetry from coordinated clocks at rest (with respect to the rotating Earth) and on the geoid is used to provide UTC, when required, on locations such as spacecraft. One of the earliest verifications of the effects of this application of general relativity, at altitudes and speeds attainable by ordinary passenger aircraft in 1971 was the Hafele-Keating experiment.

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.

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Leap second adjustments

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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 on 1 January 1958 (under another name). For this reason, UT is 'slower' than TAI.

As of 31 December 2005, TAI was ahead of UTC by 33 seconds, consisting of a 10-second offset introduced on 1 January 1972 to account for all variations between 1958 and 1971, plus an additional 22 leap seconds introduced between 1972 and 1998 followed by a single leap second added at the end of 2005. 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.

The primary dates for leap second adjustments are at the end of the Universal Day (not the local day) on June 30 and December 31. The secondary dates, which to date have been unused, are March 31 and September 30. 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 was not required between 1998 and 2005, as the deceleration of the Earth's rotation slowed temporarily in the past seven years. The last leap second, as announced by the IERS in July 2005, was on 31 December 2005. There is a proposal to redefine UTC and abolish leap seconds, such that sundials would slowly get further out-of-sync with civil time.

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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Amateur radio

Those who transmit on the amateur radio bands often log the time of their radio contacts in UTC, as transmissions can go worldwide on some frequencies. In the past, the FCC required all amateur radio operators in the United States to log their radio conversations. While maintaining a record of radio transmissions is no longer required in the U.S., many American amateur radio operators still choose to maintain a log expressing the time of their transmissions in UTC, due to the world wide reach of ham radio.

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References

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.

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