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Coordinated universal time
*** Shopping-Tip: Coordinated universal time
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| style="font-size: 95%;"|Today is {{CURRENTDAYNAME}}, {{CURRENTYEAR}}-{{CURRENTMONTH}}-{{CURRENTDAY}} ''T'' {{CURRENTTIME}} '''UTC'''.
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{{dablink|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.
{{timezones}}
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.
"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 UT. It [http://www.boulder.nist.gov/timefreq/general/misc.htm#Anchor-14550 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".
Image:Timezones.png thumb|250px|World time zones
Image:Europe time zones map en.png thumb|250px|UTC time zones in Europe
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 Time zone history. Since the
NATO phonetic alphabet and
Amateur Radio radio-amateur word for '''Z''' is "Zulu", UT is sometimes known as '''Zulu time'''.
Leap second adjustments
{{main|Leap second}}
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 (astronomy) epoch of UTC is synchronized to remain close to astronomical
Universal Time 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
Universal Time 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.
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.
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.: "[http://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf 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
*
Universal Time (UT)
*
Greenwich Mean Time (GMT)
*
Ephemeris Time
*
Sidereal time
*
Terrestrial Time
External links
-
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
-
W3C Specification about UTC Date and Time and
IETF Internet standard RFC 3339
-
Zulu Time
-
Hong Kong Time by Hong Kong Observatory
Category:Time scales
bg:Координирано универÑ?ално време
ca:Temps Universal Coordinat
cs:UTC
da:UTC
de:Koordinierte Weltzeit
el:UTC
als:UTC
es:Tiempo Universal Coordinado
eo:UTC
fr:Temps universel coordonné
ga:Am UilÃoch Comheagartha
ko:í˜‘ì • 세계시
id:UTC
io:UTC
is:UTC
it:Tempo coordinato universale
he:UTC
li:UTC
zh-min-nan:UTC
nl:Coordinated Universal Time
ja:�定世界時
nb:UTC
pl:UTC
pt:Tempo Universal Coordenado
ro:Ora universală coordonată
ru:УниверÑ?альное координированное времÑ?
simple:Coordinated Universal Time
sl:Univerzalni koordinirani Ä?as
sv:Coordinated Universal Time
sr:UTC
tt:UTC
th:เวลาพิ�ัดสา�ล
vi:Gi� phối hợp quốc tế
uk:УніверÑ?альний координований чаÑ?
tr:UTC
zh:å??调世界时
see
Coordinated Universal Time
*** Shopping-Tip: Coordinated universal time
