Coordinated Universal Time
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.
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 Universal Time. 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. The primary dates for leap second adjustments are at the end of the 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 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.
 Proposal to redefine UTC and abolish leap seconds
There is a proposal to redefine UTC and abolish leap seconds, such that sundials would slowly get further out-of-sync with civil time. See Leap second for more information.
 General information
"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 Universal Time. It may be regarded as a compromise between the English acronym "CUT" and the French acronym "TUC" (temps universel coordonné). It is sometimes erroneously expanded into "Universal Time Code".
|Hawaii-Aleutian Standard Time||UTC -10|
|Alaska Standard Time||UTC -9|
|Pacific Standard Time||UTC -8|
|Mountain Standard Time||UTC -7|
|Central Standard Time||UTC -6|
|Eastern Standard Time||UTC -5|
|Atlantic Standard Time||UTC -4|
|Greenwich Mean Time/Western European 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, Hong Kong Time,|
Chinese Standard Time
|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|
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 Amateur Radio word for Z is "Zulu", UT is sometimes known as Zulu time.
 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 of America to log their radio conversations. While maintaining a record of radio transmissions is no longer required in the USA, 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.
- 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.
- Bureau International des Poids et Mesures UTC/TAI Time Server
- The official U.S. time
- World Time Server - any location, any time
- thetimeNOW - Current time in all time zones
- United States Naval Observatory - What is Universal Time?
- International Earth Rotation Service Leap Second Updates
- Make your own UTC /Local time hardware clock
- W3C Specification about UTC Date and Time and IETF Internet standard RFC 3339
- Zulu Time
- Hong Kong Time by Hong Kong Observatory