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Coordinated Universal Time
Coordinated Universal Time (UTC) is a highprecision atomic time standard. UTC has uniform seconds defined by International Atomic Time (TAI), with leap seconds announced at irregular intervals to compensate for the earth's slowing rotation, and other discrepancies. The leap seconds allow UTC to closely track Universal Time (UT), which is a time standard based on the earth's angular rotation, rather than a uniform passage of seconds.
Time zones around the world are expressed as positive or negative offsets from UTC. In this role, UTC is also referred to as Zulu time (Z) (see Time zones below). UTC is often referred to as Greenwich Mean Time when describing time zones, although strictly UTC's atomic time scale is only approximately the same as GMT.

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Abbreviation
The International Telecommunication Union wanted Coordinated Universal Time to have a single abbreviation for all languages. English speakers and French speakers each wanted the initials 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 was used, with the verbal adjective trailing as in French.^{[1]} "UTC" can thus be read as "universal time, coordinated", although that is not the correct name in English.
"UTC" also has the benefit that it fits in with the pattern for the abbreviations of variants of Universal Time. "UT0", "UT1", "UT1R", and others exist, so appending "C" for "coordinated" to the base "UT" is very satisfactory for those who are familiar with the other types of UT.
"UTC" is sometimes erroneously expanded into "Universal Time Code".^{[2]}
Mechanism
As a time scale, UTC divides time up into days, and days into hours, minutes, and seconds. Days are conventionally identified using the Gregorian calendar, but 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.
Most UTC days contain exactly 86400 seconds, with exactly 60 seconds in each minute. Occasionally the last minute of a day has 59 or 61 seconds. So these irregular days have 86399 seconds or 86401 seconds. The irregular day lengths mean that fractional Julian days don't work properly with UTC. The intercalary seconds are known as "leap seconds".
UTC is derived from International Atomic Time (TAI), which is a time scale tracking proper time on the surface of the Earth with no reference to the rotation of the Earth. At any particular time, UTC proceeds as a linear function of TAI. From 1972 onwards UTC ticks at the same rate as TAI, but earlier (back to the 1961 start of UTC) UTC ticked at a different rate from TAI. In order to remain a close approximation of UT1 (equivalent to GMT before 1960), UTC occasionally has discontinuities where it changes from one linear function of TAI to another. These discontinuities take the form of leaps implemented by a UTC day of irregular length, and (prior to 1972) changes to the rate at which UTC ticks relative to TAI. Discontinuities in UTC have only ever occurred at the end of a Gregorian month.
The International Earth Rotation and Reference Systems Service (IERS) tracks and publishes the difference between UTC and Universal Time, DUT1 = UT1  UTC, and introduces discontinuities into UTC to keep DUT1 in the range 0.9 s < DUT1 < +0.9 s. Since 1972 the discontinuities have consisted only of a leap of one second at the end of June 30 or December 31. The IERS publishes its decision on whether to have a leap second on each of these dates a few months in advance, in Bulletin C.^{[3]} In principle leap seconds can also occur on March 31 or September 30, but the IERS has never found this necessary.
The complete definition of UTC so far, in terms of TAI, is published in the file taiutc.dat.^{[4]}
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 for more 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 twothirds 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 signals were based on UT, and hence on the rotation of the Earth.
In 1955 the caesium atomic clock was invented. This provided a form of timekeeping that was both more stable and more convenient than astronomical observations. In 1958 the International Atomic Time (TAI) service started. It was clear that basing time signals on atomic clocks would be an improvement over the prior system. However, it was widely desired to keep civil time synchronised with the Earth's rotation, and many uses of time signals (such as for navigation) relied on their closely matching Universal Time. Thus there was a need for a compromise atomicbased time scale that closely approximated UT.
UTC was initiated at the start of 1961. The TAI instant 19610101T00:00:01.422818 exactly was identified as UTC instant 19610101T00:00:00.000000 exactly, and UTC ticked exactly one second for every 1.000000015 s of TAI. Time steps and rate changes occurred every few months thereafter. These "elastic seconds" and small jumps (most commonly of 0.1 TAI seconds) were intended to permit a very close approximation of UT2, within around 0.1 s.
In 1967 the SI second was redefined in terms of the frequency supplied by a caesium atomic clock. It was soon recognised that having two types of second with different lengths, namely the UTC second and the SI second used in TAI, was a bad idea. It was thought that it would be better for time signals to maintain a consistent frequency, and that that frequency should match the SI second. Thus it would be necessary to rely on time steps alone to maintain the approximation of UT. This was tried experimentally in a service known as "Stepped Atomic Time" (SAT), which ticked at the same rate as TAI and used jumps of 0.2 s to stay synchronised with UT2.
There was also dissatisfaction with the frequent jumps in UTC (and SAT). In 1968 Louis Essen (the inventor of the caesium atomic clock) and G. M. R. Winkler both independently proposed that steps should be of 1 s only. This system was eventually approved, along with the idea of maintaining the UTC second equal to the TAI second. At the end of 1971 there was a final irregular jump of 0.107758 TAI seconds exactly, so that 19720101T00:00:00 UTC was 19720101T00:00:10 TAI exactly, making the difference between UTC and TAI an integer number of seconds. At the same time the tick rate of UTC was changed to exactly match TAI. UTC also started to track UT1 rather than UT2. Some time signals started to broadcast the DUT1 correction (UT1  UTC), for applications which required a closer approximation of UT1 than UTC now provided.
The first leap second occurred on 19720630. 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 19990101 to 20051231.
Rationale
The Earth's rotation is very slowly decreasing due to tidal braking, hence the mean solar day is increasing in length. The length of the SI second was based on the mean solar day observed between 1750 and 1892, analysed by Simon Newcomb. As a result, the SI second was exactly 1/86400 mean solar day in around 1820. In earlier centuries the mean solar day was shorter than 86400 SI seconds, and in later centuries it is longer than 86400 seconds. At the end of the twentieth century the length of the mean solar day (also known simply as "length of day" or "LOD") was approximately 86400.002 s. For this reason, UT is now 'slower' than TAI.
The excess of the LOD over the nominal 86400 s accumulates over time, causing the UTC day, initially synchronised with the mean sun, to become desynchronised and run ahead of it. At the end of the twentieth century, with the LOD at 2 ms above the nominal value, UTC ran faster than UT by 2 ms per day, getting a second ahead every 500 days. Thus a leap second was inserted once every 500 days, retarding UTC to keep it synchronised in the long term. Note that the actual rotational period varies on unpredictable factors such as tectonic motion and has to be observed rather than computed.
Take care not to confuse the difference between the length of the mean solar day and the SI day with the leap second adjustment. This error leads to the expectation that the Earth's rotation will stop in a few millennia, when there have been 86400 leap seconds. This erroneous line of reasoning confuses velocity (2 ms per day) with travelled distance (about 80 s). The correct reason for leap seconds is not the current difference between actual and nominal LOD, but rather the accumulation of this difference over a period of time.
For example, assume you start counting the seconds from the Unix epoch of 19700101T00:00:00 UTC with an atomic clock. At midnight on that day (as measured on UTC), your counter registers 0 s. After Earth has made one full rotation with respect to the mean Sun, your counter will register approximately 86400.002 s (the precise value will vary depending on plate tectonic conditions). Based on your counter, you can calculate that the date is 19700102T00:00:00 UT1. After 500 rotations, your counter will register 43 200 001 s. Since 86400 s × 500 is 43 200 000 s, you will calculate that the date is 19710516T00:00:01 UTC, while it is only 19710516T00:00:00 UT1. If you had added a leap second on December 31 1970, retarding your counter by 1 s, then the counter would have a value of 43 200 000 s at 19710516T00:00:00 UT1, and allow you to calculate the correct date.
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
As the Earth's rotation continues to slow, positive leap seconds will be required more frequently. The longterm rate of change of LOD is approximately +1.7 ms per century. At the end of the twentyfirst 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.
Sometime in the 22nd century, two leap seconds will be required every year. The current use of only the leap second opportunities in June and December will be insufficient, and the March and September options will have to be used. In the 25th century, four leap seconds will be required every year, so the current quarterly options will be insufficient. Thereafter there will need to be the possibility of leap seconds at the end of any month. In about two thousand years even that will become insufficient, and there will have to be leap seconds that are not at the end of a month.^{[5]}
In a few tens of thousands of years (the timing is very uncertain) LOD will exceed 86401 s, causing the current form of UTC to break down due to requiring more than one leap second per day. It would be possible to then continue with double leaps, but this becomes increasingly untenable.
Both the oneleapsecondpermonth and oneleapsecondperday milestones are considered (by different theorists) to mark the theoretical limit of the applicability of UTC. The actual number of leap seconds to keep track of time would become unwieldy by current standards well before these, but presumably if UTC were to continue then horological systems would be redesigned to cope with regular leap seconds much better than current systems do.
There is a proposal to redefine UTC and abolish leap seconds, such that sundials would slowly get further outofsync with civil time.^{[6]} The resulting gradual shift of the sun's movements relative to civil time is analogous to the shift of seasons relative to the yearly calendar that results from the calendar year not precisely matching the astronomical year length. This would be a major practical change in civil timekeeping, but would take effect slowly over several centuries.
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.^{[7]}
Uses
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.
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.
International broadcasts, whether they are over the internet, freetoair satellite television, or shortwave radio, always give schedules and announce times in UTC.
UTC is also the time system used in aviation. Weather reports, flight plans, air traffic control clearances, and maps all use UTC to avoid confusion about time zones and daylight saving time.
Because of time dilation, a standard clock not on the geoid, or in rapid motion, will not maintain synchronicity with UTC. Therefore, telemetry from clocks with a known relation to the geoid is used to provide UTC, when required, on locations such as spacecraft.
Because UTC is a discontinuous timescale, it is not possible to compute the exact time interval elapsed between two UTC timestamps without consulting a table that describes how many leap seconds occurred during that interval. Therefore, many scientific applications that require precise measurement of long (multiyear) intervals use TAI instead. TAI is also commonly used by systems that can not handle leap seconds.
For most practical and legaltrade 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.
Time zones
 Main articles: Time zone and List of time zones
Timezones are expressed as an offset of an integral number of minutes from UTC. The time expressed in such a timezone differs from UTC in its minute number, hour number, and possibly date, but the number of seconds is the same in all timezones. This is important when leap seconds occur. When a positive leap second occurs, it is denoted as "23:59:60" in UTC, and (for example) "20:29:60" in the Newfoundland timezone at UTC03:30. If a negative leap second occurs, such that "23:59:59" does not occur in UTC, it is "20:29:59" that is missing in the UTC03:30 timezone.
The UTC 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.
Notes
 ^ http://www.boulder.nist.gov/timefreq/general/misc.htm#Anchor14550
 ^ See, e.g., {{#invoke:citation/CS1citation CitationClass=web }}
 ^ http://hpiers.obspm.fr/iers/bul/bulc/bulletinc.dat
 ^ ftp://maia.usno.navy.mil/ser7/taiutc.dat
 ^ http://www.ucolick.org/~sla/leapsecs/utcdoomed
 ^ http://www.ucolick.org/~sla/leapsecs/
 ^ http://iraf.noao.edu/~seaman/leap/
References
 ITUR Recommendation TF.4604: Standardfrequency and timesignal 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. 915920.
 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
 Bureau International des Poids et Mesures UTC/TAI Time Server
 The official U.S. time in UTC zone using Java.
 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
 Alpha to Zulu time zones
Time offsets from Coordinated Universal Time (UTC) 12  11  10  9:30  9  8  7  6  5  4  3:30  3  2:30  2  1  0:25  UTC (0)  +0:20  +0:30  +1  +2  +3  +3:30  +4  +4:30  +4:51  +5  +5:30  +5:40  +5:45  +6  +6:30  +7  +7:20  +7:30  +8  +8:30  +8:45  +9  +9:30  +10  +10:30  +11  +11:30  +12  +12:45  +13  +13:45  +14 * Northern hemisphere countries or territories observing daylight saving time (DST)
** Southern hemisphere countries or territories observing daylight saving time (DST)