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{{Use dmy dates|date=July 2023}}
[[File:Leapsecond2016.png|thumb|right|300px|[[Screenshot]] of the [[UTC]] clock from
A '''leap second''' is a one-[[second]] adjustment that is occasionally applied to [[Coordinated Universal Time]] (UTC), to accommodate the difference between precise time ([[International Atomic Time]] (TAI), as measured by [[atomic clock]]s) and imprecise [[solar time#Mean solar time|observed solar time]] ([[UT1]]), which varies due to [[Earth rotation#Changes|irregularities]] and long-term [[ΔT (timekeeping)|slowdown]] in the [[Earth's rotation]]. The UTC time standard, widely used for international timekeeping and as the reference for [[civil time]] in most countries, uses TAI and consequently would run ahead of observed solar time unless it is reset to UT1 as needed. The leap second facility exists to provide this adjustment. The leap second was introduced in 1972. Since then, 27 leap seconds have been added to UTC, with the most recent occurring on December 31, 2016.<ref name="Martin-2024">{{Cite magazine |first=Cassie |last= Martin |date=19 January 2024 |title=50 years ago, timekeepers deployed the newly invented leap second
|url=https://www.sciencenews.org/article/50-years-ago-leap-second |department=50 Years Ago |magazine=[[Science News]] |page=4}}</ref> All have so far been positive leap seconds, adding a second to a UTC day; while it is possible for a negative leap second to be needed,
Because the Earth's rotational speed varies in response to climatic and geological events,<ref>{{cite web |url=https://www.iers.org/IERS/EN/Science/EarthRotation/EarthRotation.html |title=IERS science background |publisher=[[IERS]] |___location=Frankfurt am Main |date=2013 |access-date=6 August 2016 |url-status=live |archive-url=https://web.archive.org/web/20160829050135/https://www.iers.org/IERS/EN/Science/EarthRotation/EarthRotation.html |archive-date=29 August 2016}}</ref> UTC leap seconds are irregularly spaced and unpredictable. Insertion of each UTC leap second is usually decided about six months in advance by the [[International Earth Rotation and Reference Systems Service]] (IERS), to ensure that the difference between the UTC and UT1 readings will never exceed 0.9 seconds.<ref name="Bulletin C 49">{{cite web|url=http://hpiers.obspm.fr/eoppc/bul/bulc/bulletinc.49|title=Bulletin C 49|last=Gambis|first=Danie|date=5 January 2015|publisher=[[IERS]]|___location=Paris|access-date=5 January 2015|url-status=live|archive-url=https://web.archive.org/web/20150530123243/https://hpiers.obspm.fr/eoppc/bul/bulc/bulletinc.49|archive-date=30 May 2015}}</ref><ref>{{cite web|url=https://www.theverge.com/2015/1/7/7508651/leap-second-2015-earths-rotation-slowing|title=2015 is getting an extra second and that's a bit of a problem for the internet|author1=James Vincent|website=[[The Verge]]|date=7 January 2015|url-status=live|archive-url=https://web.archive.org/web/20170317175523/http://www.theverge.com/2015/1/7/7508651/leap-second-2015-earths-rotation-slowing|archive-date=17 March 2017}}</ref>
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Eventually, this definition too was found to be inadequate for precise time measurements, so in 1967, the [[SI second]] was again redefined as 9,192,631,770 periods of the radiation emitted by a [[caesium]]-133 atom in the transition between the two hyperfine levels of its ground state.<ref name="USNO">{{cite web|title=Leap Seconds|publisher=Time Service Department, [[United States Naval Observatory]]|url=https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-Naval-Observatory/Precise-Time-Department/Global-Positioning-System/USNO-GPS-Time-Transfer/Leap-Seconds/|access-date=19 November 2022}}</ref> That value agreed to 1 part in 10<sup>10</sup> with the astronomical (ephemeris) second then in use.<ref>[[William Markowitz|Wm Markowitz]] (1988) 'Comparisons of ET (Solar), ET (Lunar), UT and TDT', in (eds.) A K Babcock & G A Wilkins, 'The Earth's Rotation and Reference Frames for Geodesy and Geophysics', IAU Symposia #128 (1988), at pp 413–418.</ref> It was also close{{quantify|date=January 2022}} to {{frac|1|86,400}} of the mean solar day as averaged between years 1750 and 1892.
However, for the past several centuries, the length of the mean solar day has been increasing by about 1.4–1.7
When the [[Coordinated Universal Time]] (UTC) standard was instituted in 1960, based on atomic clocks, it was felt necessary to maintain agreement with UT, which, until then, had been the reference for broadcast time services. From 1960 to 1971, the rate of UTC atomic clocks was offset from a pure atomic time scale by the [[International Time Bureau|BIH]] to remain synchronized with [[UT2]], a practice known as the "rubber second".<ref>{{cite book|title=From Sundials To Atomic Clocks: Understanding Time and Frequency |first1=James |last1=Jespersen |first2=Jane |last2=Fitz-Randolph |publisher=[[National Institute of Standards and Technology]] |url=https://tf.nist.gov/general/pdf/1796.pdf |page=109 |year=1999}}</ref> The rate of UTC was decided at the start of each year, and was offset from the rate of atomic time by −150 parts per 10{{sup|10}} for 1960–1962, by −130 parts per 10{{sup|10}} for 1962–63, by −150 parts per 10{{sup|10}} again for 1964–65, and by −300 parts per 10{{sup|10}} for 1966–1971.<ref name=NBS140>{{citation |editor-last=Blair |editor-first=Byron E. |title=NBS Monograph 140: Time and Frequency: Theory and Fundamentals |url=https://nvlpubs.nist.gov/nistpubs/Legacy/MONO/nbsmonograph140.pdf |date=May 1974 |page=8}}</ref> Alongside the shift in rate, an occasional 0.1 s step (0.05 s before 1963) was needed. This predominantly frequency-shifted rate of UTC was broadcast by [[Time from NPL (MSF)|MSF]], [[WWV (radio station)|WWV]], and [[CHU (radio station)|CHU]] among other time stations. In 1966, the [[ITU-R#CCIR|CCIR]] approved "stepped atomic time" (SAT), which adjusted atomic time with more frequent 0.2 s adjustments to keep it within 0.1 s of UT2, because it had no rate adjustments.<ref>{{cite book|title=Time: From Earth Rotation to Atomic Physics|edition=second|first1=Dennis D.|last1=McCarthy|first2=P. Kenneth|last2=Seidelmann|quote=For provisional limited use, the CCIR in 1966 approved "Stepped Atomic Time," which used the atomic second with frequent 200 ms adjustments made in order to be within 0.1 s of UT2.}}</ref> SAT was broadcast by [[WWVB]] among other time stations.<ref name=NBS140/>
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In 1972, the leap-second system was introduced so that the UTC seconds could be set exactly equal to the standard SI second, while still maintaining the UTC time of day and changes of UTC date synchronized with those of UT1.<ref name="USNO"/> By then, the UTC clock was already 10 seconds behind TAI, which had been synchronized with UT1 in 1958, but had been counting true SI seconds since then. After 1972, both clocks have been ticking in SI seconds, so the difference between their displays at any time is 10 seconds plus the total number of leap seconds that have been applied to UTC as of that time; {{as of|2024|lc=on}}, 27 leap seconds have been applied to UTC, so the difference is 10 + 27 = 37 seconds. The most recent leap second was on December 31, 2016.
==Rationale==
{{See also|ΔT (timekeeping)}}▼
[[File:Deviation of day length from SI day.svg|thumb|left|Deviation of day length from SI
Leap seconds are irregularly spaced because the Earth's rotation speed changes irregularly. Indeed, the Earth's rotation is quite unpredictable in the long term, which explains why leap seconds are announced only six months in advance.▼
A [[mathematical model]] of the variations in the length of the solar day was developed by [[F. R. Stephenson]] and L. V. Morrison,<ref name="SM1995" /> based on records of [[eclipse]]s for the period 700 BC to 1623, telescopic observations of [[occultation]]s for the period 1623 until 1967 and atomic clocks thereafter. The model shows a steady increase of the mean solar day by 1.70 ms (±0.05 ms) per century, plus a periodic shift of about 4 ms amplitude and [[period (physics)|period]] of about 1,500 yr.<ref name="SM1995" /> Over the last few centuries, rate of lengthening of the mean solar day has been about 1.4 ms per century, being the sum of the periodic component and the overall rate.<ref>{{cite web |
The main reason for the slowing down of the Earth's rotation is [[tidal friction]], which alone would lengthen the day by 2.3 ms/century.<ref name="SM1995" /> Other contributing factors are the movement of the Earth's [[crust (geology)|crust]] relative to its [[planetary core|core]], changes in [[mantle convection]], and any other events or processes that cause a significant redistribution of mass. These processes change the Earth's [[moment of inertia]], affecting the rate of rotation due to the conservation of [[angular momentum]]. Some of these redistributions increase Earth's rotational speed, shorten the solar day and oppose tidal friction. For example, [[glacial rebound]] shortens the solar day by 0.6 ms/century and the [[2004 Indian Ocean earthquake and tsunami|2004 Indian Ocean earthquake]] is thought to have shortened it by 2.68 microseconds.<ref>{{cite web |last1=Cook-Anderson |first1=Gretchen |last2=Beasley |first2=Dolores |date=10 January 2005 |title=NASA Details Earthquake Effects on the Earth |url=http://www.nasa.gov/home/hqnews/2005/jan/HQ_05011_earthquake.html
It is a mistake, however, to consider leap seconds as indicators of a slowing of Earth's rotation rate; they are indicators of the accumulated difference between atomic time and time measured by Earth rotation.<ref>{{cite web |last1=Chester |first1=Geoff |date=15 June 2015 |title=Wait a second… 2015 will be a little longer |url=https://www.doncio.navy.mil/chips/ArticleDetails.aspx?ID=6471 |access-date=4 March 2021 |website=CHIPS Articles: The Department of the Navy's Information Technology Magazine
In 2021, it was reported that Earth was spinning faster in 2020 and experienced the 28 shortest days since 1960, each of which lasted less than {{val|86399.999}} seconds.<ref>{{cite web |last1=Jones |first1=Graham |last2=Bikos |first2=Konstantin |date=6 January 2021 |orig-date=2020-12-23 |title=Earth is in a hurry in 2020 |url=
==Procedure==
{| class="wikitable" style="float:right; margin-right: 0; margin-left: 1em; text-align: center;"
|+ Announced leap seconds to date<ref>{{cite web|title=TAI−UTC (1972-01-01 – 2024-06-28)|date=4 July 2023|access-date=4 July 2023|url=https://hpiers.obspm.fr/iers/bul/bulc/Leap_Second.dat}}</ref>
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The scheduling of leap seconds was initially delegated to the [[Bureau International de l'Heure]] (BIH), but passed to the International Earth Rotation and Reference Systems Service (IERS) on 1 January 1988. IERS usually decides to apply a leap second whenever the difference between UTC and UT1 approaches 0.6 s, in order to keep the difference between UTC and UT1 from exceeding 0.9 s.
The UTC standard allows leap seconds to be applied at the end of any UTC month, with first preference to June and December and second preference to March and September. {{As of|May 2023}}, all of them have been inserted at the end of either 30 June or 31 December. IERS publishes announcements every six months, whether leap seconds are to occur or not, in
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Between 1972 and 2020, a leap second has been inserted about every 21 months, on average. However, the spacing is quite irregular and apparently increasing: there were no leap seconds in the six-year interval between 1 January 1999, and 31 December
Unlike [[leap day]]s, which begin after 28 February, 23:59:59 local time,{{efn|Only the [[Gregorian calendar]]'s leap days begin after 28 February. The leap days of other calendars begin at different local times in their own years ([[Ethiopian calendar]], [[Iranian calendars]], [[Indian national calendar]], etc.).}} UTC leap seconds occur simultaneously worldwide; for example, the leap second on 31 December 2005, 23:59:60 UTC was 31 December 2005, 18:59:60 (6:59:60 p.m.) in [[U.S. Eastern Standard Time]] and 1 January 2006, 08:59:60 (a.m.) in [[Japan Standard Time]].
When it is mandated, a positive leap second is inserted between second 23:59:59 of a chosen UTC [[calendar date]] and second 00:00:00 of the following date. The definition of UTC states that the last day of December and June are preferred, with the last day of March or September as second preference, and the last day of any other month as third preference.<ref>{{cite web|url=https://www.itu.int/rec/R-REC-TF.460-6-200202-I/en|title=International Telecommunication Union Radiocommunications sector recommendation TF.460-6: Standard-frequency and time-signal emissions|access-date=9 February 2017|url-status=live|archive-url=https://web.archive.org/web/20161017185018/https://www.itu.int/rec/R-REC-TF.460-6-200202-I/en|archive-date=17 October 2016}}</ref> All leap seconds (as of 2019) have been scheduled for either 30 June or 31 December. The extra second is displayed on UTC clocks as 23:59:60. On clocks that display local time tied to UTC, the leap second may be inserted at the end of some other hour (or half-hour or quarter-hour), depending on the local time zone. A negative leap second would suppress second 23:59:59 of the last day of a chosen month so that second 23:59:58 of that date would be followed immediately by second 00:00:00 of the following date. Since the introduction of leap seconds, the mean solar day has outpaced atomic time only for very brief periods and has not triggered a negative leap second.
Recent changes to the Earth's rotation rate have made it more likely that a negative leap second will be required before the abolition of leap seconds in 2035.<ref>{{Cite web |last=Matsakis |first=Demetrios |date=September 21, 2022 |title=Will we have a negative leap second? |url=https://www.gps.gov/cgsic/meetings/2022/matsakis.pdf |access-date=3 June 2024 |website=gps.gov}}</ref><ref>{{Cite journal |last=Agnew |first=Duncan Carr |date=April 2024 |title=A global timekeeping problem postponed by global warming |url=https://www.nature.com/articles/s41586-024-07170-0 |journal=Nature |language=en |volume=628 |issue=8007 |pages=333–336 |doi=10.1038/s41586-024-07170-0 |pmid=38538793 |bibcode=2024Natur.628..333A |issn=1476-4687|url-access=subscription }}</ref>
==Future==
▲{{See also|ΔT (timekeeping)}}
▲[[File:Deviation of day length from SI day.svg|thumb|left|Deviation of day length from SI based day with shorter days resulting from faster planetary rotation.]]
▲Leap seconds are irregularly spaced because the Earth's rotation speed changes irregularly. Indeed, the Earth's rotation is quite unpredictable in the long term, which explains why leap seconds are announced only six months in advance.
▲A [[mathematical model]] of the variations in the length of the solar day was developed by [[F. R. Stephenson]] and L. V. Morrison,<ref name=SM1995/> based on records of [[eclipse]]s for the period 700 BC to 1623, telescopic observations of [[occultation]]s for the period 1623 until 1967 and atomic clocks thereafter. The model shows a steady increase of the mean solar day by 1.70 ms (±0.05 ms) per century, plus a periodic shift of about 4 ms amplitude and [[period (physics)|period]] of about 1,500 yr.<ref name=SM1995/> Over the last few centuries, rate of lengthening of the mean solar day has been about 1.4 ms per century, being the sum of the periodic component and the overall rate.<ref>{{cite web|url=http://www.ucolick.org/~sla/leapsecs/dutc.html|title=Extrapolations of the difference (TI – UT1)|work=ucolick.org|date=8 June 2011|author=Steve Allen|access-date=29 February 2016|url-status=live|archive-url=https://web.archive.org/web/20160304193859/http://www.ucolick.org/~sla/leapsecs/dutc.html|archive-date=4 March 2016}}</ref>
▲The main reason for the slowing down of the Earth's rotation is [[tidal friction]], which alone would lengthen the day by 2.3 ms/century.<ref name=SM1995/> Other contributing factors are the movement of the Earth's [[crust (geology)|crust]] relative to its [[planetary core|core]], changes in [[mantle convection]], and any other events or processes that cause a significant redistribution of mass. These processes change the Earth's [[moment of inertia]], affecting the rate of rotation due to the conservation of [[angular momentum]]. Some of these redistributions increase Earth's rotational speed, shorten the solar day and oppose tidal friction. For example, [[glacial rebound]] shortens the solar day by 0.6 ms/century and the [[2004 Indian Ocean earthquake and tsunami|2004 Indian Ocean earthquake]] is thought to have shortened it by 2.68 microseconds.<ref>{{cite web|last1=Cook-Anderson |first1=Gretchen |last2=Beasley |first2=Dolores |url=http://www.nasa.gov/home/hqnews/2005/jan/HQ_05011_earthquake.html|title=NASA Details Earthquake Effects on the Earth|publisher=[[National Aeronautics and Space Administration]] (press release).|date=10 January 2005|url-status=live|archive-url=https://web.archive.org/web/20110127163105/http://www.nasa.gov/home/hqnews/2005/jan/HQ_05011_earthquake.html|archive-date=27 January 2011}}</ref>
▲It is a mistake, however, to consider leap seconds as indicators of a slowing of Earth's rotation rate; they are indicators of the accumulated difference between atomic time and time measured by Earth rotation.<ref>{{cite web |last1=Chester |first1=Geoff |title=Wait a second… 2015 will be a little longer |url=https://www.doncio.navy.mil/chips/ArticleDetails.aspx?ID=6471 |website=CHIPS Articles: The Department of the Navy's Information Technology Magazine |date=15 June 2015|access-date=4 March 2021}}</ref> The plot at the top of this section shows that in 1972 the average length of day was approximately {{val|86400.003}} seconds and in 2016 it was approximately {{val|86400.001}} seconds, indicating an overall increase in Earth's rotation rate over that time period. Positive leap seconds were inserted during that time because the annual average length of day remained greater than {{val|86400}} SI seconds, not because of any slowing of Earth's rotation rate.<ref>{{cite web |last1=Plait |first1=Phil |title=Followup: Leap Seconds |url=https://www.discovermagazine.com/the-sciences/followup-leap-seconds |website=Discover Magazine: Bad Astronomy |date=31 December 2008|access-date=5 March 2021}}</ref>
▲In 2021, it was reported that Earth was spinning faster in 2020 and experienced the 28 shortest days since 1960, each of which lasted less than {{val|86399.999}} seconds.<ref>{{cite web |last1=Jones |first1=Graham |last2=Bikos |first2=Konstantin |title=Earth is in a hurry in 2020 |url= https://www.timeanddate.com/time/earth-faster-rotation.html |website=timeanddate.com |date=6 January 2021 |orig-date=2020-12-23|access-date=6 March 2021}}</ref> This caused engineers worldwide to discuss a negative leap second and other possible timekeeping measures, some of which could eliminate leap seconds.<ref>{{cite news |last1=Knapton |first1=Sarah |title=The Earth is spinning faster now than at any time in the past half century |url=https://www.telegraph.co.uk/news/2021/01/04/earth-spinning-faster-now-time-past-half-century/ |archive-url=https://ghostarchive.org/archive/20220112/https://www.telegraph.co.uk/news/2021/01/04/earth-spinning-faster-now-time-past-half-century/ |archive-date=12 January 2022 |url-access=subscription |url-status=live |access-date=11 February 2021 |work=[[The Daily Telegraph]] |date=4 January 2021}}{{cbignore}}</ref> The shortest day ever recorded was 29 June 2022, at 1.59 milliseconds less than 24 hours.<ref name="NHM 270324">{{cite news |url=https://www.nhm.ac.uk/discover/news/2024/march/climate-change-causing-days-get-longer-slowing-down-earth.html |title=Climate change is causing days to get longer by slowing down the Earth |first=James |last=Ashworth |date= 27 March 2024 |publisher=Natural History Museum |___location=London}}</ref> In a 2024 paper published in [[Nature (journal)|''Nature'']], Duncan Agnew of the [[Scripps Institution of Oceanography]] projects that the water from increasing [[ice cap]] melting will migrate to the equator and thus cause the rate of rotation to slow down again.<ref name="NHM 270324" />
{{More citations needed|section|date=December 2023}}
The TAI and UT1 time scales are precisely defined, the former by atomic clocks (and thus independent of Earth's rotation) and the latter by astronomical observations (that measure actual planetary rotation and thus the solar time at the
The irregularity and unpredictability of UTC leap seconds is problematic for several areas, especially [[computing]] (see [[#Issues created by insertion (or removal) of leap seconds|below]]). With increasing requirements for [[timestamp]] accuracy in systems such as process automation and [[high-frequency trading]],<ref>{{Cite news| title = Time Split to the Nanosecond Is Precisely What Wall Street Wants |newspaper =[[The New York Times]]| access-date = 13 December 2022| url = https://www.nytimes.com/2018/06/29/technology/computer-networks-speed-nasdaq.html |date = 29 June 2018}}</ref> this raises a number of issues. Consequently, the long-standing practice of inserting leap seconds is under review by the relevant international standards body.<ref>{{Cite web |last=Dwyer |first=Colin |date=29 December 2016 |title=With A Leap Second, 2016 Promises To Linger Just A Little Bit Longer |url=https://www.npr.org/sections/thetwo-way/2016/12/29/507422729/with-a-leap-second-2016-promises-to-linger-just-a-little-bit-longer |url-status=live |archive-url=https://web.archive.org/web/20230102214327/https://www.npr.org/sections/thetwo-way/2016/12/29/507422729/with-a-leap-second-2016-promises-to-linger-just-a-little-bit-longer |archive-date=2 January 2023 |access-date=24 February 2023 |website=[[NPR]]}}</ref>
===
{{anchor|Proposal to abolish leap seconds}}
On 5 July 2005, the Head of the Earth Orientation Center of the IERS sent a notice to IERS Bulletins C and D subscribers, soliciting comments on a U.S. proposal before the ITU-R Study Group 7's WP7-A to eliminate leap seconds from the UTC broadcast standard before 2008 (the [[ITU-R]] is responsible for the definition of UTC).{{efn|''[[The Wall Street Journal]]'' noted that the proposal was considered by a U.S. official at the time to be a "private matter internal to the ITU."<ref>{{cite news |url=https://www.wsj.com/articles/SB112258962467199210?mod=home_page_one_us |title=Why the U.S. Wants To End the Link Between Time and Sun |newspaper=The Wall Street Journal |access-date=31 October 2017 |url-status=live |archive-url=https://web.archive.org/web/20171107010404/https://www.wsj.com/articles/SB112258962467199210?mod=home_page_one_us |archive-date=7 November 2017}}</ref>}} It was expected to be considered in November 2005, but the discussion has since been postponed.<ref>{{cite news |url=http://news.bbc.co.uk/2/hi/science/nature/4420084.stm |title=Leap second talks are postponed |work=[[BBC News]] |access-date=31 October 2017 |url-status=live |archive-url=https://web.archive.org/web/20171107031708/http://news.bbc.co.uk/2/hi/science/nature/4420084.stm |archive-date=7 November 2017}}</ref> Under the proposal, leap seconds would be technically replaced by leap hours as an attempt to satisfy the legal requirements of several ITU-R member nations that civil time be astronomically tied to the Sun.
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A number of objections to the proposal have been raised. P. Kenneth Seidelmann, editor of the Explanatory Supplement to the Astronomical Almanac, wrote a letter lamenting the lack of consistent public information about the proposal and adequate justification.<ref>{{cite mailing list |url=https://lists.igs.org/pipermail/igsmail/2005/006563.html |title=UTC redefinition or change |author=P. Kenneth Seidelmann |mailing-list=IGS Mail}}</ref> In an [[op-ed]] for ''[[Science News]]'', Steve Allen of the [[University of California, Santa Cruz]] said that the process has a large impact on astronomers.<ref>{{cite magazine |last=Cowen |first=Ron |date=22 April 2006 |title=To Leap or Not to Leap: Scientists debate a timely issue |url=https://www.sciencenews.org/article/leap-or-not-leap |url-status=live |magazine=[[Science News]] |archive-url=https://web.archive.org/web/20230526024544/https://www.sciencenews.org/article/leap-or-not-leap |archive-date=26 May 2023 |access-date=26 May 2023}}</ref>
At the 2014 General Assembly of the [[International Union of Radio Scientists]] (URSI), Demetrios Matsakis, the [[United States Naval Observatory]]'s Chief Scientist for Time Services, presented the reasoning in favor of the redefinition and rebuttals to the arguments made against it.<ref>{{cite web |url=http://tycho.usno.navy.mil/papers/ts-2014/Matsakis-LeapSecondComments.URSI-2014.pdf |title=Comments on the Debate over the Proposal to Redefine UTC |author1=Demetrios Matsakis |date=18 August 2014 |access-date=31 October 2017 |url-status=
The United States formulated its position on this matter based upon the advice of the [[National Telecommunications and Information Administration]]<ref>{{cite web|url=https://www.ntia.doc.gov/files/ntia/publications/ai_1.14_usa_proposal_2014-02-06_0.pdf|title=United States Proposals, Proposal for the Work of the Conference, Agenda Item 1.14|publisher=[[National Telecommunications and Information Administration]]}}</ref> and the [[Federal Communications Commission]] (FCC), which solicited comments from the general public.<ref>{{cite web|url=https://apps.fcc.gov/edocs_public/attachmatch/DA-14-88A1.pdf|title=FCC Seeks Comment On Recommendations Approved By The Advisory Committee For The 2015 World Radiocommunication Conference|publisher=[[Federal Communications Commission]]|date=28 January 2014|url-status=live|archive-url=https://web.archive.org/web/20140729075437/https://apps.fcc.gov/edocs_public/attachmatch/DA-14-88A1.pdf|archive-date=29 July 2014}}</ref> This position is in favor of the redefinition.<ref>{{cite web|url=https://www.ntia.doc.gov/files/ntia/publications/sitt-stit-357221-v1-citel_presentation_for_regional_meetings_on_wrc-15-r2.ppt|title=Preliminary Views and Proposals Regarding WRC-15 Agenda Items|publisher=[[Organization of American States]]|format=PPT|url-status=live|archive-url=https://web.archive.org/web/20140729090447/http://www.ntia.doc.gov/files/ntia/publications/sitt-stit-357221-v1-citel_presentation_for_regional_meetings_on_wrc-15-r2.ppt|archive-date=29 July 2014}}</ref>{{efn|The FCC has posted its received comments, which can be found using their search engine for proceeding 04–286 and limiting the "received period" to those between 27 January and 18 February 2014, inclusive.<ref>{{cite web|url=http://apps.fcc.gov/ecfs/comment_search/execute?proceeding=04-286&applicant=&lawfirm=&author=&disseminated.minDate=&disseminated.maxDate=&received.minDate=1%2F27%2F14&received.maxDate=2%2F18%2F14&dateCommentPeriod.minDate=&dateCommentPeriod.maxDate=&dateReplyComment.minDate=&dateReplyComment.maxDate=&address.city=&address.state.stateCd=&address.zip=&daNumber=&fileNumber=&bureauIdentificationNumber=&reportNumber=&submissionTypeId=&__checkbox_exParte=true|title=Search for Filings Results|work=fcc.gov|url-status=live|archive-url=https://web.archive.org/web/20150701090036/http://apps.fcc.gov/ecfs/comment_search/execute?proceeding=04-286&applicant=&lawfirm=&author=&disseminated.minDate=&disseminated.maxDate=&received.minDate=1%2F27%2F14&received.maxDate=2%2F18%2F14&dateCommentPeriod.minDate=&dateCommentPeriod.maxDate=&dateReplyComment.minDate=&dateReplyComment.maxDate=&address.city=&address.state.stateCd=&address.zip=&daNumber=&fileNumber=&bureauIdentificationNumber=&reportNumber=&submissionTypeId=&__checkbox_exParte=true|archive-date=1 July 2015}}</ref>}}
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ITU World Radiocommunication Conference 2023 (WRC-23), which was held in Dubai (United Arab Emirates) from 20 November to 15 December 2023 formally recognized the [https://www.bipm.org/en/cgpm-2022/resolution-4 Resolution 4] of the 27th CGPM (2022) which decides that the maximum value for the difference (UT1-UTC) will be increased in, or before, 2035.<ref>{{cite web |url=https://www.bipm.org/en/-/2023-12-12-wrc-dubai |title=ITU-R and BIPM work together at the World Radiocommunication Conference |website=BIPM}}</ref>
==Problems==
[[File:ChronyControl screenshot.webp|thumb|upright=1.5|Screenshot of [[Chrony]]Control on [[macOS]], showing an insert second announcement by NTP on 30 June 2015.]]
===
To compute the elapsed time in seconds between two given UTC dates requires the consultation of a table of leap seconds, which needs to be updated whenever a new leap second is announced. Since leap seconds are known only 6 months in advance, time intervals for UTC dates further in the future cannot be computed.
===Missing
Although [[BIPM]] announces a leap second 6 months in advance, most time distribution systems ([[SNTP]], [[IRIG-B]], [[Precision Time Protocol|PTP]]) announce leap seconds at most 12 hours in advance,{{citation needed|date=October 2019}}<ref>{{Cite tech report |title=A Resilient Architecture for the Realization and Distribution of Coordinated Universal Time to Critical Infrastructure Systems in the United States |url=https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=933488 |date=November 2021 |doi=10.6028/NIST.TN.2187|doi-access=free }}</ref> sometimes only in the last minute and some even not at all ([[DNP3]]).{{citation needed|date=October 2019}}
===Implementation differences===
Not all clocks implement leap seconds in the same manner. Leap seconds in [[Unix time]] are commonly implemented by repeating 23:59:59 or adding the time-stamp 23:59:60. [[Network Time Protocol]] (
===Textual representation
The textual representation of a leap second is defined by BIPM as "23:59:60". There are programs that are not familiar with this format and may report an error when dealing with such input.
===Binary representation
Most computer operating systems and most time distribution systems represent time with a binary counter indicating the number of seconds elapsed since an arbitrary [[epoch]]; for instance, since {{nowrap|1970-01-01}} 00:00:00 in POSIX machines or since {{nowrap|1900-01-01}} 00:00:00 in NTP. This counter does not count positive leap seconds, and has no indicator that a leap second has been inserted, therefore two seconds in sequence will have the same counter value. Some computer operating systems, in particular Linux, assign to the leap second the counter value of the preceding, 23:59:59 second ({{nowrap|59–59–0}} sequence), while other computers (and the IRIG-B time distribution) assign to the leap second the counter value of the next, 00:00:00 second ({{nowrap|59–0–0}} sequence).{{citation needed|date=October 2019}} Since there is no standard governing this sequence, the timestamp of values sampled at exactly the same time can vary by one second. This may explain flaws in time-critical systems that rely on timestamped values.<ref>{{Cite journal |last1=Benzler |first1=Justus |last2=Clark |first2=Samuel J. |date=30 March 2005 |title=Toward a Unified Timestamp with explicit precision |journal=[[Demographic Research (journal)|Demographic Research]] |volume=12 |issue=6 |pages=107–140 |doi=10.4054/DemRes.2005.12.6 |issn=1435-9871 |pmc=2854819 |pmid=20396403}}</ref>
===Others===
Several models of global navigation satellite receivers have software flaws associated with leap seconds:
* Some older versions of Motorola Oncore VP, UT, GT, and M12 GPS receivers had a software bug that would cause a single timestamp to be off by a day if no leap second was scheduled for 256 weeks. On 28 November 2003, this happened. At midnight, the receivers with this firmware reported 29 November 2003, for one second and then reverted to 28 November 2003.<ref>{{cite web|url=http://www.leapsecond.com/notes/leapsec256.htm|title=256-Week Leap Second Bug|date=2 July 2013|url-status=live|archive-url=https://web.archive.org/web/20160304002759/http://www.leapsecond.com/notes/leapsec256.htm|archive-date=4 March 2016}}</ref><ref>{{cite web|url=http://compgroups.net/comp.protocols.time.ntp/motorola-oncore-receivers-and-leap-se/287130|title=Motorola Oncore receivers and Leap Second bug|date=2 July 2013|url-status=usurped|archive-url=https://web.archive.org/web/20130118233907/http://compgroups.net/comp.protocols.time.ntp/motorola-oncore-receivers-and-leap-se/287130|archive-date=18 January 2013}}</ref>
* Older Trimble GPS receivers had a software flaw that would insert a leap second immediately after the [[List of GPS satellites|GPS constellation]] started broadcasting the next leap second insertion time (some months in advance of the actual leap second), rather than waiting for the next leap second to happen. This left the receiver's time off by a second in the interim.<ref>{{cite web|url=http://www.guralp.com/howtos/leap-second-problem-with-older-gps-receivers.shtml|title=Leap-second problem with older GPS receivers|date=19 November 2014|url-status=live|archive-url=https://web.archive.org/web/20141129055128/http://www.guralp.com/howtos/leap-second-problem-with-older-gps-receivers.shtml|archive-date=29 November 2014}}</ref><ref>{{cite web|url=http://www.spirent.com/Blogs/Positioning/2015/May/How_Leap_Seconds_Can_Interfere_with_GNSS_Receivers|title=How Leap Seconds Can Interfere with GNSS Receivers|date=13 May 2015|url-status=live|archive-url=https://web.archive.org/web/20160306014132/http://www.spirent.com/Blogs/Positioning/2015/May/How_Leap_Seconds_Can_Interfere_with_GNSS_Receivers|archive-date=6 March 2016}}</ref>
* Older Datum/Symmetricom
* Four different brands of navigational receivers that use data from [[BeiDou]] satellites were found to implement leap seconds one day early.<ref>{{cite web |date=3 March 2015 |title=BeiDou Numbering Presents Leap-Second Issue |url=https://www.gpsworld.com/beidou-numbering-presents-leap-second-issue/ |publisher=GPS World}}</ref> This was traced to a bug related to how the BeiDou protocol numbers the days of the week.
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Due to a software error, the UTC time broadcast by the NavStar GPS system was incorrect by about 13 microseconds on 25–26 January 2016.<ref>{{cite news|url=https://elpromatime.com/wp-content/uploads/2022/08/US_AirForce_OfficialPressRelease.pdf |title=Air Force Official Press Release – GPS Ground System Anomaly}}</ref><ref>{{cite news |url=https://tf.nist.gov/general/pdf/2886.pdf |title=The effects of the January 2016 UTC offset anomaly on GPS-controlled clocks monitored at NIST |first1=Jian |last1=Yao |first2=Michael A. |last2=Lombardi |first3=Andrew N. |last3=Novick |first4=Bijunath |last4=Patla |first5=Jeff A. |last5=Sherman |first6=Victor |last6=Zhang}}</ref>
==Workarounds
The most obvious workaround is to use the TAI scale for all operational purposes and convert to UTC for human-readable text. UTC can always be derived from TAI with a suitable table of leap seconds. The [[Society of Motion Picture and Television Engineers]] (SMPTE) video/audio industry standards body selected TAI for deriving timestamps of media.<ref>{{cite web|url=http://www.ieee802.org/1/files/public/docs2013/asbt-briscoe-timing-and-sync-SMPTE-0513.pdf|title=Network-Based Timing and Synchronization|author=Paul Briscoe|date=14 May 2013}}</ref>
IEC/IEEE 60802 (Time sensitive networks) specifies TAI for all operations. Grid automation is planning to switch to TAI for global distribution of events in electrical grids. [[Bluetooth mesh networking]] also uses TAI.<ref>{{Cite web|url=https://www.bluetooth.org/docman/handlers/downloaddoc.ashx?doc_id=429634|title=Mesh Model Bluetooth® Specification|date=13 July 2017|website=Bluetooth Technology Website|format=PDF download|access-date=14 December 2019}} See sections 5.1.1 and A.1.</ref>
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