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((Short description|Time scale with correction))
((refimprove|date=December 2020))
[[Image:Leapsecond.ut1-utc.svg|thumb|right|400px|Graph showing the difference in seconds between [[UT1]] and [[UTC]] over time. A vertical transition indicates the use of a [[leap second]]. The red portion of the graph indicates predicted values.]]

'''DUT1''' (sometimes also written '''DUT''') is a time correction equal to the difference between [[Universal Time]] ([[UT1]]), which is defined by [[Earth's rotation]], and [[Coordinated Universal Time]] (UTC), which is defined by a network of precision [[atomic clock]]s.

:DUT1 = UT1 − UTC

UTC is maintained via [[leap second]]s, such that DUT1 remains within the range −0.9 s < DUT1 < +0.9 s. The reason for this correction is partly that the rate of rotation of the Earth is not constant, due to [[tidal braking]] and the redistribution of mass within the Earth, including its oceans and atmosphere, and partly because the [[second|SI second]] (as now used for [[UTC]]) was, when adopted, a little shorter than the current value of the second of mean solar time.((efn|(1) In "The Physical Basis of the Leap Second", by D D McCarthy, C Hackman and R A Nelson, in Astronomical Journal, vol.136 (2008), pages 1906–1908, it is stated (page 1908), that "the SI second is equivalent to an older measure of the second of UT1, which was too small to start with and further, as the duration of the UT1 second increases, the discrepancy widens." :(2) In the late 1950s, the cesium standard was used to measure both the current mean length of the second of mean solar time ([[UT2]]) (result: ((val|9192631830)) cycles) and also the second of [[ephemeris time]] (ET) (result: ((val|9192631770|20)) cycles), see [http://www.leapsecond.com/history/1968-Metrologia-v4-n4-Essen.pdf "Time Scales", by L. Essen], in Metrologia, vol.4 (1968), pp.161–165, on p.162. As is well known, the ((val|9192631770)) figure was chosen for the [[second|SI second]].  L Essen in the same 1968 article (p.162) stated that this "seemed reasonable in view of the variations in UT2".))

Daily observed values of DUT1 for the past week, and daily forecast values for the coming year, are published by [[IERS]] [https://datacenter.iers.org/data/latestVersion/bulletinA.txt Bulletin A]. Final values are published in the monthly [[IERS]] [https://datacenter.iers.org/data/latestVersion/bulletinB.txt Bulletin B].

Several [[time signal service]]s broadcast values of DUT1. [[CHU (radio station)|CHU]] (Canada), [[HLA (radio station)|HLA]] (South Korea), [[Time from NPL|MSF]] (United Kingdom), and [[WWV (radio station)|WWV]] (United States) transmit DUT1 with 0.1&nbsp;s precision. In Russia, [[RWM]], [[RTZ (radio station)|RTZ]] and the longwave [[RBU (radio station)|RBU]] transmit DUT1 with 0.1&nbsp;s precision and an additional correction dUT1 in 0.02&nbsp;s increments.<ref>((cite web |title=Time Signal Transmitter |url=https://www.meinbergglobal.com/english/glossary/time-signal-transmitter.htm |website=www.meinbergglobal.com |access-date=21 March 2022))</ref>

==See also==
* ((anl|ΔT (timekeeping)|ΔT))

==Notes==
((notelist))

==References==
((reflist))
* ITU-R Recommendation TF.460-4: Standard-frequency and time-signal emissions. [[International Telecommunication Union]].

==External links==
* [https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html IERS Bulletins], including Bulletin A – latest and past versions, and graphical plots
* Latest version of [https://datacenter.iers.org/data/latestVersion/6_BULLETIN_A_V2013_016.txt Bulletin A]
*[http://maia.usno.navy.mil/ IERS Rapid Service/Prediction Center via the US Naval Observatory] ([https://web.archive.org/web/20191003033945/http://maia.usno.navy.mil:80/ Archived version] available for non-US readers)
* http://scienceworld.wolfram.com/astronomy/Time.html
((Time measurement and standards))

[[Category:Time scales]]

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