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Coordinated Universal Time

coordinated universal time, coordinated universal time clock
Coordinated Universal Time French: Temps universel coordonné, abbreviated as UTC, is the primary time standard by which the world regulates clocks and time It is within about 1 second of mean solar time at 0° longitude; it does not observe daylight saving time It is one of several closely related successors to Greenwich Mean Time GMT For most purposes, UTC is considered interchangeable with GMT, but GMT is no longer precisely defined by the scientific community

The first Coordinated Universal Time was informally adopted on January 1, 1960

The system was adjusted several times, including a brief period where time coordination radio signals broadcast both UTC and "Stepped Atomic Time SAT" until a new UTC was adopted in 1970 and implemented in 1972 This change also adopted leap seconds to simplify future adjustments This CCIR Recommendation 460 "stated that a carrier frequencies and time intervals should be maintained constant and should correspond to the definition of the SI second; b step adjustments, when necessary, should be exactly 1 s to maintain approximate agreement with Universal Time UT; and c standard signals should contain information on the difference between UTC and UT"

A number of proposals have been made to replace UTC with a new system that would eliminate leap seconds, but no consensus has yet been reached

The current version of UTC is defined by International Telecommunications Union Recommendation ITU-R TF460-6, Standard-frequency and time-signal emissions and is based on International Atomic Time TAI with leap seconds added at irregular intervals to compensate for the slowing of Earth's rotation Leap seconds are inserted as necessary to keep UTC within 09 seconds of universal time, UT1 See the "Current number of leap seconds" section for the number of leap seconds inserted to date


  • 1 Etymology
  • 2 Uses
  • 3 Mechanism
    • 31 Time zones
    • 32 Daylight saving time
  • 4 History
    • 41 Current number of leap seconds
  • 5 Rationale
  • 6 Future
  • 7 See also
  • 8 References
    • 81 Notes
    • 82 Bibliography
  • 9 External links


The official abbreviation for Coordinated Universal Time is UTC This abbreviation arose from a desire by the International Telecommunication Union and the International Astronomical Union to use the same abbreviation in all languages English speakers originally proposed CUT for "coordinated universal time", while French speakers proposed TUC for "temps universel coordonné" The compromise that emerged was UTC, which conforms to the pattern for the abbreviations of the variants of Universal Time UT0, UT1, UT2, UT1R, etc


Time zones around the world are expressed using positive or negative offsets from UTC, as in the list of time zones by UTC offset

The westernmost time zone uses UTC−12, being twelve hours behind UTC; the easternmost time zone, theoretically, uses UTC+12, being twelve hours ahead of UTC In 1995, the island nation of Kiribati moved those of its atolls in the Line Islands from UTC-10 to UTC+14 so that the country would all be on the same day

UTC is used in many internet and World Wide Web standards The Network Time Protocol, designed to synchronise the clocks of computers over the internet, encodes times using the UTC system Computer servers, online services and other entities that rely on having a universally accepted time use UTC as it is more specific than GMT If only limited precision is needed, clients can obtain the current UTC from a number of official internet UTC servers For sub-microsecond precision, clients can obtain the time from satellite signals

UTC is also the time standard used in aviation, eg, for flight plans and air traffic control clearances Weather forecasts and maps all use UTC to avoid confusion about time zones and daylight saving time The International Space Station also uses UTC as a time standard

Amateur radio operators often schedule their radio contacts in UTC, because transmissions on some frequencies can be picked up by many time zones

UTC is also used in digital tachographs used on large goods vehicles LGV under EU and AETR rules


UTC divides time into days, 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 The number of seconds in a minute is usually 60, but with an occasional leap second, it may be 61 or 59 instead Thus, in the UTC time scale, the second and all smaller time units millisecond, microsecond, etc are of constant duration, but the minute and all larger time units hour, day, week, etc are of variable duration Decisions to introduce a leap second are announced at least six months in advance in "Bulletin C" produced by the International Earth Rotation and Reference Systems Service The leap seconds cannot be predicted far in advance due to the unpredictable rate of rotation of the Earth

Nearly all UTC days contain exactly 86,400 SI seconds with exactly 60 seconds in each minute However, because the mean solar day is slightly longer than 86,400 SI seconds, occasionally the last minute of a UTC day is adjusted to have 61 seconds The extra second is called a leap second It accounts for the grand total of the extra length about 2 milliseconds each of all the mean solar days since the previous leap second The last minute of a UTC day is permitted to contain 59 seconds to cover the remote possibility of the Earth rotating faster, but that has not yet been necessary The irregular day lengths mean that fractional Julian days do not work properly with UTC

Since 1972, UTC is calculated by subtracting the accumulated leap seconds from International Atomic Time TAI, which is a coordinate time scale tracking notional proper time on the rotating surface of the Earth the geoid In order to maintain a close approximation to UT1 equivalent to GMT, UTC occasionally has discontinuities where it changes from one linear function of TAI to another These discontinuities take the form of leap seconds implemented by a UTC day of irregular length Discontinuities in UTC have occurred only at the end of June or December, although there is provision for them to happen at the end of March and September as well as a second preference 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 interval −09 s, +09 s

As with TAI, UTC is only known with the highest precision in retrospect Users who require an approximation in real time must obtain it from a time laboratory, which disseminates an approximation using techniques such as GPS or radio time signals Such approximations are designated UTCk, where k is an abbreviation for the time laboratory The time of events may be provisionally recorded against one of these approximations; later corrections may be applied using the International Bureau of Weights and Measures BIPM monthly publication of tables of differences between canonical TAI/UTC and TAIk/UTCk as estimated in real time by participating laboratories See the article on International Atomic Time for details

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 those of spacecraft

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 By extension, it is not possible to compute the duration of a time interval that ends in the future and may encompass an unknown number of leap seconds for example, the number of TAI seconds between "now" and 2099-12-31 23:59:59 Therefore, many scientific applications that require precise measurement of long multi-year intervals use TAI instead TAI is also commonly used by systems that cannot handle leap seconds GPS time always remains exactly 19 seconds behind TAI neither system is affected by the leap seconds introduced in UTC

For most common and legal-trade purposes, the fractional second difference between UTC and UT GMT is inconsequentially small Greenwich Mean Time is the legal standard in Britain during the winter, and this notation is familiar to and used by the population

Time zones

Main articles: Time zone and Lists of time zones See also: UTC offset and List of UTC time offsets "Zulu time" redirects here For the album by Caspar Brötzmann and Page Hamilton, see Zulutime

Time zones are usually defined as differing from UTC by an integer number of hours, although the laws of each jurisdiction would have to be consulted if sub-second accuracy was required Several jurisdictions have established time zones that differ by an integer number of half-hours or quarter-hours from UT1 or UTC

Current civil time in a particular time zone can be determined by adding or subtracting the number of hours and minutes specified by the UTC offset, which ranges from UTC−12:00 in the west to UTC+14:00 in the east see List of UTC time offsets

The time zone using UTC is sometimes denoted UTC±00:00 or by the letter Z—a reference to the equivalent nautical time zone GMT, which has been denoted by a Z since about 1950 Time zones were identified by successive letters of the alphabet and the Greenwich time zone was marked by a Z as it was the point of origin The letter also refers to the "zone description" of zero hours, which has been used since 1920 see time zone history Since the NATO phonetic alphabet word for Z is "Zulu", UTC is sometimes known as "Zulu time" This is especially true in aviation, where "Zulu" is the universal standard This ensures all pilots regardless of location are using the same 24-hour clock, thus avoiding confusion when flying between time zones See the list of military time zones for letters used in addition to Z in qualifying time zones other than Greenwich

On electronic devices that only allow the current time zone to be configured using maps or city names, UTC can be selected indirectly by selecting Reykjavík, Iceland, which is always on UTC and does not use daylight saving time

Daylight saving time

Main article: Daylight saving time

UTC does not change with a change of seasons, but local time or civil time may change if a time zone jurisdiction observes daylight saving time summer time For example, local time on the east coast of the United States is five hours behind UTC during winter, but four hours behind while daylight saving is observed there


At the 1884 International Meridian Conference held in Washington, DC, the local mean solar time at the Royal Observatory, Greenwich in England was chosen to define the Universal day, counted from 0 hours at mean midnight This agreed with civil Greenwich Mean Time GMT, used on the island of Great Britain since 1847 In contrast, astronomical GMT began at mean noon, 12 hours after mean midnight of the same date until 1 January 1925, whereas nautical GMT began at mean noon, 12 hours before mean midnight of the same date, at least until 1805 in the Royal Navy, but persisted much later elsewhere because it was mentioned at the 1884 conference In 1884, the Greenwich Meridian was used for two-thirds of all charts and maps as their Prime Meridian In 1928, the term Universal Time UT was introduced by the International Astronomical Union 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 1956, the US National Bureau of Standards and US Naval Observatory started to develop atomic frequency time scales; by 1959, these time scales were used in generating the WWV time signals, named for the shortwave radio station that broadcasts them In 1960, the US Naval Observatory, the Royal Greenwich Observatory, and the UK National Physical Laboratory coordinated their radio broadcasts so time steps and frequency changes were coordinated, and the resulting time scale was informally referred to as "Coordinated Universal Time"

In a controversial decision, the frequency of the signals was initially set to match the rate of UT, but then kept at the same frequency by the use of atomic clocks and deliberately allowed to drift away from UT When the divergence grew significantly, the signal was phase shifted stepped by 20 ms to bring it back into agreement with UT Twenty-nine such steps were used before 1960

In 1960, the ephemeris second was defined as In 1958, data was published linking the frequency for the caesium transition, newly established, with the ephemeris second The ephemeris second is the duration of time that, when used as the independent variable in the laws of motion that govern the movement of the planets and moons in the solar system, causes the laws of motion to accurately predict the observed positions of solar system bodies Within the limits of observing accuracy, ephemeris seconds are of constant length, as are atomic seconds This publication allowed a value to be chosen for the length of the atomic second that would work properly with the celestial laws of motion

In 1961, the Bureau International de l'Heure began coordinating the UTC process internationally but the name Coordinated Universal Time was not adopted by the International Astronomical Union until 1967 Time steps occurred every few months thereafter, and frequency changes at the end of each year The jumps increased in size to 100 ms This UTC was intended to permit a very close approximation to UT2

In 1967, the SI second was redefined in terms of the frequency supplied by a caesium atomic clock The length of second so defined was practically equal to the second of ephemeris time This was the frequency that had been provisionally used in TAI since 1958 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 200 ms 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 exactly 0107758 TAI seconds, so that 1 January 1972 00:00:00 UTC was 1 January 1972 00:00:10 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 requiring a closer approximation of UT1 than UTC now provided

Current number of leap seconds

The first leap second occurred on 30 June 1972 Since then, leap seconds have occurred on average about once every 19 months, always on 30 June or 31 December As of July 2015, there have been 26 leap seconds in total, all positive, putting UTC 36 seconds behind TAI


Graph showing the difference DUT1 between UT1 and UTC in seconds Vertical segments correspond to leap seconds

Earth's rotational speed is very slowly decreasing because of tidal deceleration; this increases the length of the mean solar day The length of the SI second was calibrated on the basis of the second of ephemeris time and can now be seen to have a relationship with the mean solar day observed between 1750 and 1892, analysed by Simon Newcomb As a result, the SI second is close to 1/86400 of a mean solar day in the mid‑19th century In earlier centuries, the mean solar day was shorter than 86,400 SI seconds, and in more recent centuries it is longer than 86,400 seconds Near the end of the 20th century, the length of the mean solar day also known simply as "length of day" or "LOD" was approximately 86,4000013 s For this reason, UT is now "slower" than TAI by the difference or "excess" LOD of 13 ms/day

The excess of the LOD over the nominal 86,400 s accumulates over time, causing the UTC day, initially synchronised with the mean sun, to become desynchronised and run ahead of it Near the end of the 20th century, with the LOD at 13 ms above the nominal value, UTC ran faster than UT by 13 ms per day, getting a second ahead roughly every 800 days Thus, leap seconds were inserted at approximately this interval, retarding UTC to keep it synchronised in the long term The actual rotational period varies on unpredictable factors such as tectonic motion and has to be observed, rather than computed

Just as adding a leap day every four years does not mean the year is getting longer by one day every four years, the insertion of a leap second every 800 days does not indicate that the mean solar day is getting longer by a second every 800 days It will take about 50,000 years for a mean solar day to lengthen by one second at a rate of 2 ms/cy, where cy means century This rate fluctuates within the range of 17–23 ms/cy While the rate due to tidal friction alone is about 23 ms/cy, the uplift of Canada and Scandinavia by several metres since the last Ice Age has temporarily reduced this to 17 ms/cy over the last 2,700 years The correct reason for leap seconds, then, is not the current difference between actual and nominal LOD, but rather the accumulation of this difference over a period of time: Near the end of the 20th century, this difference was about 1/800 of a second per day; therefore, after about 800 days, it accumulated to 1 second and a leap second was then added

In the graph of DUT1 above, the excess of LOD above the nominal 86,400 s corresponds to the downward slope of the graph between vertical segments The slope became shallower in the 2000s decade, because of a slight acceleration of 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


See also: Leap second

As the Earth's rotation continues to slow, positive leap seconds will be required more frequently The long-term rate of change of LOD is approximately +17 ms per century At the end of the 21st century, LOD will be roughly 86,400004 s, requiring leap seconds every 250 days Over several centuries, the frequency of leap seconds will become problematic

Some time 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 be insufficient, and there will have to be leap seconds that are not at the end of a month In a few tens of thousands of years the timing is uncertain, LOD will exceed 86,401 s, causing UTC to require more than one leap second per day

In April 2001, Rob Seaman of the National Optical Astronomy Observatory proposed that leap seconds be allowed to be added monthly rather than twice yearly

There is a proposal to redefine UTC and abolish leap seconds, such that sundials would slowly get further out of sync with civil time 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 tropical year length This would be a major practical change in civil timekeeping, but would take effect slowly over several centuries UTC and TAI would be more and more ahead of UT; it would coincide with local mean time along a meridian drifting slowly eastward reaching Paris and beyond Thus, the time system would lose its fixed connection to the geographic coordinates based on the IERS meridian The difference between UTC and UT could reach 05 hour after the year 2600 and 65 hours around 4600

ITU‑R Study Group 7 and Working Party 7A were unable to reach consensus on whether to advance the proposal to the 2012 Radiocommunications Assembly; the chairman of Study Group 7 elected to advance the question to the 2012 Radiocommunications Assembly 20 January 2012, but consideration of the proposal was postponed by the ITU until the World Radio Conference in 2015, convening on 2 November

The possibility of suppressing the leap second was considered in November 2015 at the World Radiocommunication Conference WRC-15, which is the international regulatory body which defines Coordinated Universal Time No decision to suppress leap seconds was reached; the issue will be studied further and reconsidered in 2023

See also

  • Geography portal
  • Time portal
  • Ephemeris time
  • IERS Reference Meridian
  • ISO 8601
  • List of UTC timing centers
  • Mars Time Coordinated MTC
  • Terrestrial Time
  • World Radiocommunication Conference



  1. ^ Guinot 2011, p S181
  2. ^ a b c "COORDINATED UNIVERSAL TIME UTC CCTF/09-32" PDF Bureau International des Poids et Mesures p 3 Retrieved 30 October 2016 
  3. ^ ITU Radiocommunication Assembly 2002
  4. ^ Time Service Dept c 2009
  5. ^ National Institute of Standards and Technology 2012
  6. ^ National Institute of Standards and Technology 2011
  7. ^ IAU resolutions 1976
  8. ^ How NTP Works 2011
  9. ^ Aviation Time 2006
  10. ^ Horzepa 2010
  11. ^ ITU Radiocommunication Assembly 2002, p 3
  12. ^ International Earth Rotation and Reference Systems Service 2011
  13. ^ McCarthy & Seidelmann 2009, p 229
  14. ^ McCarthy & Seidelmann 2009, chapter 4
  15. ^ History of TAI-UTC c 2009
  16. ^ McCarthy & Seidelmann 2009, pp 217, 227–231
  17. ^ McCarthy & Seidelmann 2009, p 209
  18. ^ Time nd
  19. ^ Langley 1999
  20. ^ Seidelmann 1992, p 7
  21. ^ Military & Civilian Time Designations nd
  22. ^ Williams 2005
  23. ^ Iceland 2011
  24. ^ Standard time 2010
  25. ^ Howse 1997, pp 133–137
  26. ^ McCarthy & Seidelmann 2009, pp 10–11
  27. ^ a b McCarthy & Seidelmann 2009, pp 226–227
  28. ^ Arias, Guinot & Quinn 2003
  29. ^ "The second is the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom" CGPM 1960, Resolution 9, as quoted in Bureau International des Poids et Mesures 2006, 133
  30. ^ a b Markowitz et al 1958
  31. ^ Nelson & McCarthy 2005, p 15
  32. ^ Nelson et al 2001, p 515
  33. ^ a b Markowitz 1988
  34. ^ McCarthy & Seidelmann 2009, p 227
  35. ^ Essen 1968, pp 161–5
  36. ^ Seidelmann 1992, pp 85–87
  37. ^ Nelson, Lombardi & Okayama 2005, p 46
  38. ^ Bulletin C 2014
  39. ^ McCarthy & Seidelmann 2009, p 87
  40. ^ McCarthy & Seidelmann 2009, p 54
  41. ^ McCarthy & Seidelmann 2009, p 230 Average for period from 1 January 1991 through 1 January 2009 Average varies considerably depending on what period is chosen
  42. ^ Stephenson & Morrison 1995
  43. ^ a b Allen 2011a
  44. ^ Rob Seaman 9 Apr 2001 "Upgrade, don't degrade" Archived from the original on 2 June 2013 Retrieved 2015-09-10 
  45. ^ Allen 2011b
  46. ^ Irvine 2008
  47. ^ Seidelmann & Seago 2011, p S190
  48. ^ Leap decision postponed 2012
  49. ^ "ITU World Radiocommunication Conference set for Geneva, 2–27 November 2015" International Telecommunications Union 2015 Retrieved November 3, 2015 
  50. ^ "Coordinated Universal Time UTC to retain 'leap second'" International Telecommunications Union 2015-11-19 Retrieved November 19, 2015 


  • Allan, David W; Ashby, Neil; Hodge, Clifford C 1997 The Science of Timekeeping Hewlett-Packard  Application Note
  • Allen, Steve 2011a "UTC is doomed" Retrieved 18 July 2011 
  • Allen, Steve 2011b "UTC might be redefined without Leap Seconds" Retrieved 18 July 2011 
  • Arias, E F; Guinot, B; Quinn, T J 29 May 2003 Rotation of the Earth and Time scales PDF ITU-R Special Rapporteur Group Colloquium on the UTC Time Scale 
  • "Aviation Time" AOPA's Path to Aviation Aircraft Owners and Pilots Association 2006 Retrieved 17 July 2011 
  • "Bulletin C" International Earth Rotation and Reference Systems Service 16 January 2014 
  • Essen, L 1968 "Time Scales" PDF Metrologica 4 4: 161–5 Bibcode:1968Metro4161E doi:101088/0026-1394/4/4/003 Retrieved 18 October 2008 
  • Finkleman, David; Allen, Steve; Seago, John; Seaman, Rob; Seidelmann, P Kenneth 2011 "The Future of Time: UTC and the Leap Second" American Scientist 99 July–August 2011: 312 arXiv:11063141v1 doi:101511/2011911 
  • Guinot, Bernard August 2011 "Solar time, legal time, time in use" Metrologica 48 4: S181–185 Bibcode:2011Metro48S181G doi:101088/0026-1394/48/4/S08 
  • "History of TAI-UTC" Time Service Dept, US Naval Observatory c 2009 Retrieved 4 January 2009 
  • Horzepa, Stan 17 September 2010 "Surfin': Time for Ham Radio" American Radio Relay League Retrieved 24 October 2011 
  • Howse, Derek 1997 Greenwich Time and the Longitude London: Philip Wilson ISBN 0-85667-468-0 
  • "How NTP Works" NTP: The Network Time Protocol 28 July 2011  See heading "NTP Timescale and Data Formats"
  • "IAU resolutions adopted at the XVIth General Assembly, Grenoble, France, 1976" PDF 1976  Resolution no 3 by Commissions 4 Ephemerides/Ephémérides and 31 Time/L'Heure near the end of the document "recommend that the following notations be used in all languages", UT0i, UT1i, UT2i, UTC, UTCi, UT, where i is institution "i"
  • "Iceland" 2011 
  • International Bureau of Weights and Measures 10 October 2011 "Circular T" 285 
  • International Earth Rotation and Reference Systems Service 19 July 2011 "IERS Bulletins" 
  • Irvine, Chris 18 December 2008 "Scientists propose 'leap hour' to fix time system" The Telegraph 
  • ITU Radiocommunication Assembly 2002 "Standard-frequency and time-signal emissions" PDF International Telecommunications Union Retrieved 2 August 2011 
  • Langley, Richard B 20 January 1999 "A Few Facts Concerning GMT, UT, and the RGO" Retrieved 17 July 2011 
  • "Leap second decision is postponed" BBC News 19 January 2012 
  • McCarthy, Dennis D July 1991 "Astronomical Time" PDF Proc IEEE 79 7: 915–920 doi:101109/584967 
  • McCarthy, Dennis D; Seidelmann, P Kenneth 2009 TIME From Earth Rotation to Atomic Physics Weinheim: Wiley VCH ISBN 978-3-527-40780-4 
  • Markowitz, W; Hall, R; Essen, L; Parry, J August 1958 "Frequency of caesium in terms of Ephemeris Time" PDF Physical Review Letters 1 3: 105–7 Bibcode:1958PhRvL1105M doi:101103/PhysRevLett1105 Retrieved 18 October 2008 
  • Markowitz, Wm 1988 "Comparisons of ET Solar, ET Lunar, UT and TDT" In Babcock, A K; Wilkins, G A The Earth's Rotation and Reference Frames for Geodesy and Geophysics: Proceedings of the 128th Symposium of the International Astronomical Union, held in Coolfont, West Virginia, USA, 20–24 October 1986 Dordrecht: Kluwer Academic Publishers pp 413–418 Bibcode:1988IAUS128413M 
  • "Military & Civilian Time Designations" wwp 
  • National Institute of Standards and Technology 18 January 2011 "Frequently asked questions FAQ" Retrieved 17 July 2011 
  • National Institute of Standards and Technology 19 March 2012 "Frequently asked questions FAQ" 
  • Nelson, GK; Lombardi, MA; Okayama, DT 2005 "NIST Time and Frequency Radio Stations: WWV, WWVH, and WWVB" PDF National Institute of Standards and Technology Special Publication 250-67 Archived PDF from the original on 26 June 2008 
  • Nelson, Robert A; McCarthy, Dennis D 13 September 2005 Coordinated Universal Time UTC and the Future of the Leap Second Civil GPS Interface Committee United States Coast Guard Archived from the original on 29 April 2011 
  • Nelson, Robert A; McCarthy, Dennis D; Malys, S; Levine, J; Guinot, B; Fliegel, H F; Beard, R L; Bartholomew, T R 2001 "The leap second: its history and possible future" PDF Metrologia 38 6: 509–529 Bibcode:2001Metro38509N doi:101088/0026-1394/38/6/6 
  • Seidelmann, P Kenneth; Seago, John H August 2011 "Time scales, their users, and leap seconds" Metrologia 48 4: S186–S194 Bibcode:2011Metro48S186S doi:101088/0026-1394/48/4/S09 
  • Seaman, Rob 2003 "A Proposal to Upgrade UTC" Retrieved 18 July 2011 
  • Seidelmann, PK 1992 Explanatory Supplement to the Astronomical Almanac Sausalito, CA: University Science Books 
  • Stephenson, F R; Morrison, L V 1995 "Long-term fluctuations in the Earth's rotation: 700 BC to AD 1990" Philosophical Transactions of the Royal Society A 351 1695: 165–202 Bibcode:1995RSPTA351165S doi:101098/rsta19950028 
  • "Standard time" US Code Legal Information Institute 2010  Title 15, Chapter 6, Subchapter IX
  • "TF460-4: Standard-frequency and time-signal emissions" PDF International Telecommunication Union 1986 Annex I 
  • "Time" International Bureau of Weights and Measures nd Retrieved 22 May 2013 
  • Time Service Dept c 2009 "Leap Seconds" United States Naval Observatory Retrieved 17 July 2011 
  • United States Naval Observatory "Universal Time" Retrieved 10 October 2013 
  • "Universal Time" Oxford Dictionaries: British and World English Oxford University Press Retrieved 6 August 2014 
  • Williams, Jack 17 May 2005 "Understanding and using Zulu time" USA Today Retrieved 25 February 2007 

External links

  • Current UTC time
  • Definition of Coordinated Universal Time in German law–ZeitG §1 3
  • International Earth Rotation Service; list of differences between TAI and UTC from 1961 to present
  • US Naval Observatory: Systems of Time
  • W3C Specification about UTC Date and Time and IETF Internet standard RFC 3339, based on ISO 8601
  • Standard of time definition: UTC, GPS, LORAN and TAI
  • What is in a name On the term Coordinated Universal Time

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