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== Abbreviation ==
{| class="wikitable" style="margin: .5em .5em .5em .5em; float: right;"
|+ Compromise abbreviation
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"UTC" has been erroneously expanded into "Universal Time Code", "Universal Time Convention", or "Universal Time Coordinated."<ref>{{cite web|url=http://goespoes.gsfc.nasa.gov/poes/acronym/u.html|title=POES Acronyms and Abbreviations|accessdate=2006-07-26}}</ref>
== Mechanism ==
As a [[time standard|time scale]], UTC divides up time into days, hours, minutes, and seconds. Days are conventionally identified using the [[Gregorian calendar]], but [[Julian day|Julian Day Numbers]] can also be used. Each day contains 24 hours and each hour contains 60 minutes, but the number of seconds in a minute is slightly variable.
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As with TAI, UTC is only known with the highest precision in retrospect. The [[International Bureau of Weights and Measures]] (BIPM) publishes monthly tables of differences between canonical TAI/UTC and TAI/UTC as estimated in real time by participating laboratories. (See the article on [[TAI|International Atomic Time]] for details.)
== History ==
Originally, the local time at the [[Royal Observatory, Greenwich]], England was chosen as standard at the [[1884]] [[International Meridian Conference]], leading to the widespread use of Greenwich Mean Time ([[GMT]]) 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]]. In [[1929]], the term Universal Time (UT) was introduced to refer to GMT with the day starting at midnight. Until the 1950s, broadcast [[time signal]]s were based on UT, and hence on the rotation of the Earth.
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The first leap second occurred on [[1972-06-30]]. Since then leap seconds have occurred on average once every 18 months, always on [[June 30]] or [[December 31]]. [[As of 2006]] there have been 23 leap seconds in total, all positive, putting UTC 33 seconds behind TAI. It seems unlikely that a negative leap second will ever occur, but there is a small chance of one due to the acceleration of the Earth's crust in the 2000s. This acceleration has already led to the longest ever period without a leap second, from [[1999-01-01]] to [[2005-12-31]].
== Rationale ==
[[Акс:Leapsecond.ut1-utc.svg|thumb|left|250px|Graph showing the difference [[DUT1]] between UT1 and UTC. Vertical segments correspond to leap seconds.]]
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In the graph of [[DUT1]] above, the excess of LOD above the nominal 86400 s corresponds to the downward slope of the graph between vertical segments. (Note that the slope became shallower in the 2000s, due to a slight acceleration of the Earth's crust temporarily shortening the day.) Vertical position on the graph corresponds to the accumulation of this difference over time, and the vertical segments correspond to leap seconds introduced to match this accumulated difference. Leap seconds are timed to keep DUT1 within the vertical range depicted by this graph. The frequency of leap seconds therefore corresponds to the slope of the diagonal graph segments, and thus to the excess LOD.
== Future ==
{{seealso|leap second}}
As the Earth's rotation continues to slow, positive leap seconds will be required more frequently. The long-term [[derivative|rate of change]] of LOD is approximately +1.7 ms per century. At the end of the [[21st century|twenty-first century]] LOD will be roughly 86400.004 s, requiring leap seconds every 250 days. Over several centuries, the frequency of leap seconds will become problematic.
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There is also a proposal that the present form of UTC could be improved to track UT1 more closely, by allowing greater freedom in scheduling leap seconds.<ref>{{cite web|url=http://iraf.noao.edu/~seaman/leap/|title=A Proposal to Upgrade UTC<!--INSERT TITLE-->|accessdate=2007-06-02}}</ref>
== Uses ==
UTC is the time system used for many [[Internet]] and [[World Wide Web]] standards. In particular, The [[Network Time Protocol]], which is designed to synchronize the clocks of many computers over the Internet (usually to that of a known accurate atomic clock), uses UTC.
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For most common and legal-trade purposes, the fractional second difference between UTC and UT ([[GMT]]) is inconsequentially small, so UTC is often called GMT, for example by the [[BBC]], although that usage is not technically correct.<ref>{{cite web|url=http://www.apparent-wind.com/gmt-explained.html|title=A Few Facts Concerning GMT, UT, and the RGO|accessdate=2007-08-19}}</ref>
== Time zones ==
{{main|Time zone|List of time zones}}
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The UTC time zone is sometimes denoted by the letter '''Z''' – a reference to the equivalent nautical time zone ([[GMT]]), which has been denoted by a '''Z''' since about 1950. The letter also refers to the "zone description" of '''zero''' hours, which has been used since [[1920]]. See [[time zone#History|time zone history]]. Since the [[NATO phonetic alphabet]] and [[amateur radio]] word for '''Z''' is "Zulu", UTC is sometimes known as '''Zulu time'''. This is especially true in aviation, where Zulu is the universal standard.<ref>[http://wwp.greenwichmeantime.com/info/timezone.htm Military & Civilian Time Designations]</ref> This ensures all pilots regardless of location are using the same [[24-hour clock]], thus avoiding confusion when flying between time zones.<ref>Williams, Jack. [http://www.usatoday.com/weather/zulu.htm Understanding and using Zulu time] ''USA Today''. Accessed [[2007-02-25]].</ref><ref> Smith, Sandi. [http://www.sandismith.com/mathbook/chap1_7.html Zulu Time]. Sandismith Keynotes and Workshops. Accessed [[2007-02-07]].</ref>
== See also ==
* [[Ephemeris Time]]
* [[Sidereal time]]
* [[Terrestrial Time]]
* [[ISO 8601]]
== Bibliography ==
* [[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, [[1991|July 1991]], pp. 915-920.
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* David W. Allan, Neil Ashby, Clifford C. Hodge: The Science of Timekeeping. [[Hewlett-Packard|Hewlett Packard]] [[Application note|Application Note]] 1289, 1997.
== References ==
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== External links ==
* [http://www.bipm.org/en/scientific/tai/time_server.html Bureau International des Poids et Mesures UTC/TAI Time Server]
* [http://www.time.gov/timezone.cgi?UTC/s/0/java The official U.S. time in UTC zone] using [[Java Platform|Java]].
* [http://www.worldtimeserver.com/ World Time Server – any location, any time]
* [http://www.thetimenow.com/ thetimeNOW – Current time in all time zones]
* [http://aa.usno.navy.mil/faq/docs/UT.php United States Naval Observatory – ''What is Universal Time?'']
* [http://hpiers.obspm.fr/eoppc/bul/bulc/bulletinc.dat International Earth Rotation Service Leap Second Updates]
* [http://www.w3.org/TR/NOTE-datetime W3C Specification about UTC Date and Time] and [[IETF]] [[Internet standard]] [[RFC 3339]], based on [[ISO 8601]]
* [http://www.grc.nasa.gov/WWW/MAEL/ag/zulu.htm Zulu Time]
* [http://www.hko.gov.hk/gts/time/worldtime2.htm Hong Kong Time by Hong Kong Observatory]
* [http://wwp.greenwichmeantime.com/info/timezone.htm Alpha to Zulu time zones]
* [http://www.cl.cam.ac.uk/~mgk25/time/zeitgesetz.en.html Definition of Coordinated Universal Time in German law – ZeitG §1 (3)]
* [http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf The Science of Timekeeping] -- by David W. Allan of [[Allan Variance]] fame
[[af:Gekoördineerde Universele Tyd]]
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