View source for Leap second

{{Short description|Intermittent adjustment to UTC}}
{{Use dmy dates|date=July 2023}}
[[File:Leapsecond2016.png|thumb|[[Screenshot]] of the [[UTC]] clock from time.gov during the leap second on 31 December  2016.]]

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 |magazine=[[Science News]] }}</ref> All have so far been positive leap seconds, adding a second to a UTC day; while a negative leap second is theoretically possible, it has not yet occurred.

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]] |date=2013 |access-date=6 August 2016}}</ref> UTC leap seconds are irregularly spaced and not precisely predictable. The decision to insert a leap second is made by the [[International Earth Rotation and Reference Systems Service]] (IERS), typically about six months in advance, to ensure that the difference between UTC and UT1 does not exceed ±0.9 seconds.<ref name="BulletinC49">{{cite web|url=http://hpiers.obspm.fr/eoppc/bul/bulc/bulletinc.49|title=Bulletin C 49|last=Gambis|first=Daniel|date=5 January 2015|publisher=[[IERS]]|access-date=5 January 2015}}</ref>

This practice has proven disruptive, particularly in modern digital systems that require continuous and precise timestamping. Since not all systems implement leap-second adjustments uniformly, some computational, networking, and satellite-based systems may encounter anomalies.<ref>{{cite journal |last1=Finkleman |first1=David |last2=Allen |first2=Steve |last3=Seago |first3=John |last4=Seaman |first4=Rob |last5=Seidelmann |first5=P. Kenneth |year=2011 |title=The Future of Time: UTC and the Leap Second |url=https://www.americanscientist.org/article/the-future-of-time-utc-and-the-leap-second |journal=[[American Scientist]] |volume=99 |issue=4 |pages=312–319}}</ref>

== Phase-out and future of leap seconds ==

At its 27th meeting in November 2022, the [[General Conference on Weights and Measures]] (CGPM) passed Resolution 4, directing that the maximum permitted difference between UT1 and UTC be increased "in, or before, 2035."<ref>{{Cite web|title=Resolution 4 of the 27th CGPM (2022): On the use and future development of UTC|url=https://www.bipm.org/en/-/resolution-cgpm-27-4|access-date=2025-11-17}}</ref> The goal is to reduce the frequency of leap seconds by widening the tolerance, rather than continuing to insert them indefinitely.

The resolution instructs the CIPM (International Committee for Weights and Measures) to develop, in consultation with the International Telecommunication Union (ITU) and other stakeholders, a plan to implement a new maximum (UT1–UTC) value by 2035 that would ensure UTC continuity for at least a century.<ref>{{Cite web|title=CGPM 2022 Resolution 4 text and plan for continuous UTC|url=https://www.bipm.org/documents/20126/220989730/CIPM112/205c84da-6d72-8253-71c8-cf91f585c601|access-date=2025-11-17}}</ref> The 28th CGPM, scheduled for 2026, is expected to review and potentially approve this new limit.<ref>{{Cite web|title=IETF NTP presentation “Future of Leap Seconds”|url=https://www.ietf.org/proceedings/117/slides/slides-117-ntp-future-of-leap-seconds-00.pdf|access-date=2025-11-17}}</ref>

== Implications and technical debate ==

Many proponents of the change argue that removing frequent leap seconds will improve the stability and reliability of time dissemination systems (e.g., telecommunications, GNSS, data networks), avoiding the disruptions caused by inserting or removing a second unpredictably.<ref>{{Cite web|title=Network‑crashing leap seconds to be abandoned by 2035, for at least a century|url=https://arstechnica.com/science/2022/11/network-crashing-leap-seconds-to-be-abandoned-by-2035-for-at-least-a-century/|access-date=2025-11-17}}</ref>

According to the BIPM resolution text and technical studies, proposed alternative strategies include gradual “smear” adjustments (spreading the additional time over hours or days) or very rare “leap minute” insertions, rather than the current unpredictable leap-second model.<ref>{{Cite web|title=IGS Technical Report on continuous UTC and leap seconds|url=https://files.igs.org/pub/resource/technical_reports/2022_techreport.pdf|access-date=2025-11-17}}</ref>

While the focus is on removing frequent leap seconds, the link between UTC and Earth's rotation (UT1) would not be permanently broken: the resolution explicitly preserves that connection, just with a more relaxed bound.<ref>{{Cite web|title=Global timekeepers vote to scrap leap second by 2035|url=https://www.ibtimes.com/global-timekeepers-vote-scrap-leap-second-2035-3637869|access-date=2025-11-17}}</ref>

== Criticism and counterpoints ==

Some critics warn that increasing the permissible divergence between UTC and UT1 could, over very long timescales, weaken civil time’s alignment with solar time (i.e., what “noon” feels like vs. real solar noon), though BIPM officials argue that the increase will be gradual and manageable.<ref>{{Cite web|last1=Graham |first1=Jones |title=The End of the Leap Second?|url=https://www.timeanddate.com/news/astronomy/end-of-leap-seconds-2022|access-date=2025-11-17}}</ref>

Opponents also caution that the “smear second” solution may not be ideal for all systems: the lack of a sudden, well-defined leap makes it harder for some time‑sensitive protocols to maintain compatibility without additional engineering.<ref>{{Cite web|title=Network‑crashing leap seconds to be abandoned by 2035, for at least a century|url=https://arstechnica.com/science/2022/11/network-crashing-leap-seconds-to-be-abandoned-by-2035-for-at-least-a-century/|access-date=2025-11-17}}</ref>

== Long-term outlook ==

If implemented as planned, the change would mean no more leap-second insertions for many decades, reducing disruptions in high-precision digital systems.
The new (UT1–UTC) limit is expected to be revisited periodically; the resolution mandates a review mechanism so that the tolerance does not diverge uncontrollably as Earth's rotation evolves.<ref>{{Cite web|title=Draft plan for review at 28th CGPM (2026) and beyond|url=https://www.bipm.org/documents/20126/220989730/CIPM112/205c84da-6d72-8253-71c8-cf91f585c601|access-date=2025-11-17}}</ref>

==History==
[[File:Leapsecond.ut1-utc.svg|thumb|Graph showing the difference between UT1 and UTC. Vertical segments correspond to leap seconds.]]
In about AD 140, [[Ptolemy]], the Alexandrian astronomer, [[sexagesimal]]ly subdivided both the mean solar day and the [[true solar day]] to at least six places after the sexagesimal point, and he used simple fractions of both the equinoctial hour and the seasonal hour, none of which resemble the modern second.<ref name=Ptolemy>{{cite book|author=Ptolemy|author-link=Ptolemy|author2=G. J. Toomer|others=Toomer, G. J.|title=Ptolemy's Alemagest|year = 1998|publisher=Princeton University Press|location=Princeton, New Jersey|isbn=978-0-691-00260-6|pages=6–7, 23, 211–216}}</ref>    Muslim scholars, including [[al-Biruni]] in 1000, subdivided the mean solar day into 24 equinoctial hours, each of which was subdivided sexagesimally, that is into the units of minute, second, third, fourth and fifth, creating the modern second as {{nowrap|1= {{frac|1|60}} of {{frac|1|60}} of {{frac|1|24}} = {{frac|1|86,400}}}} of the mean solar day in the process.<ref name="al-Biruni">{{cite book|author=al-Biruni|author-link=al-Biruni|others=Sachau, C. Edward|title=The chronology of ancient nations: an English version of the Arabic text of the Athâr-ul-Bâkiya of Albîrûnî, or "Vestiges of the Past"|url=https://books.google.com/books?id=pFIEAAAAIAAJ|year=1879|publisher=Oriental Translation Fund of Great Britain & Ireland|pages=141–149, 158, 408, 410|url-status=live|archive-url=https://web.archive.org/web/20171114142935/https://books.google.com/books?id=pFIEAAAAIAAJ|archive-date=14 November 2017}} Used for mean new moons, both in [[Hebrew calendar]] cycles and in equivalent astronomical cycles.</ref> With this definition, the second was proposed in 1874 as the base unit of time in the [[CGS system of units]].<ref>{{cite book|title=Illustrations of the centimetre-gramme-second (C.G.S.) system of units|url=https://archive.org/details/illustrationsce00evergoog|year=1875|publisher=Taylor and Francis|page=[https://archive.org/details/illustrationsce00evergoog/page/n99 83]|first1=J. D.|last1=Everett|author-link1=Joseph David Everett}}</ref> Soon afterwards [[Simon Newcomb]] and others discovered that Earth's rotation period varied irregularly,<ref name="Pearce">{{cite journal | last1 = Pearce | first1 = J. A. | year = 1928 | title = The Variability of the Rotation of the Earth | journal = Journal of the Royal Astronomical Society of Canada | volume = 22 | pages = 145–147 | bibcode = 1928JRASC..22..145P }}</ref> so in 1952, the [[International Astronomical Union]] (IAU) defined the second as a fraction of the [[sidereal year]]. In 1955, considering the [[tropical year]] to be more fundamental than the sidereal year, the IAU redefined the second as the fraction {{frac|1|31,556,925.975}} of the 1900.0 [[mean tropical year]]. In 1956, a slightly more precise value of {{frac|1|31,556,925.9747}} was adopted for the definition of the second by the [[International Committee for Weights and Measures]], and in 1960 by the [[General Conference on Weights and Measures]], becoming a part of the [[International System of Units]] (SI).<ref name=Supplement>{{cite book|title=Explanatory Supplement to the Astronomical Almanac|url=https://books.google.com/books?id=WBiqdNy_2KIC|year=1992|publisher=University Science Books|location=Mill Valley, California|isbn=0-935702-68-7|pages=79–80|editor1-first=P. Kenneth|editor1-last=Seidelmann|url-status=live|archive-url=https://web.archive.org/web/20171114142935/https://books.google.com/books?id=WBiqdNy_2KIC|archive-date=14 November 2017}}</ref>

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&nbsp;[[millisecond|ms]] per century, depending on the averaging time.<ref>DD McCarthy and AK Babcock (1986), "The Length of the Day Since 1658", Phys. Earth Planet Inter., No. 44, pp. 281–292</ref><ref>RA Nelson, DD McCarthy, S Malys, J Levine, B Guinot, HF Fliegel, RL Beard, and TR Bartholomew, (2001) "The Leap Second: its History and Possible Future" (2001), Metrologia 38, pp. 509–529</ref><ref name=SM1995>{{cite journal | last1 = Stephenson | first1 = F.R. | last2 = Morrison | first2 = L.V. | year = 1995 | title = Long-term fluctuations in the Earth's rotation: 700 BC to AD 1990 | bibcode = 1995RSPTA.351..165S | journal = Philosophical Transactions of the Royal Society of London A | volume = 351 | issue = 1695| pages = 165–202 | doi=10.1098/rsta.1995.0028| s2cid = 120718607 }}</ref> By 1961, the mean solar day was already a millisecond or two longer than {{val|86400}} SI seconds.<ref>{{cite journal | last1 = McCarthy | first1 = D D | last2 = Hackman | first2 = C | last3 = Nelson | first3 = R A | year = 2008 | title = The Physical Basis of the Leap Second | url = https://apps.dtic.mil/sti/pdfs/ADA489427.pdf | archive-url = https://web.archive.org/web/20210312034304/https://apps.dtic.mil/sti/pdfs/ADA489427.pdf | url-status = live | archive-date = 12 March 2021 | journal = Astronomical Journal | volume = 136 | issue = 5 | pages = 1906–1908 | doi = 10.1088/0004-6256/136/5/1906 | bibcode = 2008AJ....136.1906M | doi-access = free | access-date = 26 February 2022}}</ref> Therefore, time standards that change the date after precisely {{val|86400}} SI seconds, such as the [[International Atomic Time]] (TAI), would become increasingly ahead of time standards tied to the mean solar day, such as [[Universal Time]] (UT).

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&nbsp;s step (0.05&nbsp;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&nbsp;s adjustments to keep it within 0.1&nbsp;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&nbsp;ms adjustments made in order to be within 0.1&nbsp;s of UT2.}}</ref> SAT was broadcast by [[WWVB]] among other time stations.<ref name=NBS140/>

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-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&nbsp;ms (±0.05&nbsp;ms) per century, plus a periodic shift of about 4&nbsp;ms amplitude and [[period (physics)|period]] of about 1,500&nbsp;yr.<ref name="SM1995" /> Over the last few centuries, rate of lengthening of the mean solar day has been about 1.4&nbsp;ms per century, being the sum of the periodic component and the overall rate.<ref>{{cite web |author=Steve Allen |date=8 June 2011 |title=Extrapolations of the difference (TI – UT1) |url=http://www.ucolick.org/~sla/leapsecs/dutc.html |url-status=live |archive-url=https://web.archive.org/web/20160304193859/http://www.ucolick.org/~sla/leapsecs/dutc.html |archive-date=4 March 2016 |access-date=29 February 2016 |work=ucolick.org}}</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&nbsp;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&nbsp;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 |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 |publisher=[[National Aeronautics and Space Administration]] (press release).}}</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 |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}}</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 |date=31 December 2008 |title=Followup: Leap Seconds |url=https://www.discovermagazine.com/the-sciences/followup-leap-seconds |access-date=5 March 2021 |website=Discover Magazine: Bad Astronomy}}</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 |date=6 January 2021 |orig-date=2020-12-23 |title=Earth is in a hurry in 2020 |url=https://www.timeanddate.com/time/earth-faster-rotation.html |access-date=6 March 2021 |website=timeanddate.com}}</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 |date=4 January 2021 |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/ |url-access=subscription |url-status=live |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 |access-date=11 February 2021 |work=[[The Daily Telegraph]]}}{{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 |last=Ashworth |first=James |date=27 March 2024 |title=Climate change is causing days to get longer by slowing down the Earth |url=https://www.nhm.ac.uk/discover/news/2024/march/climate-change-causing-days-get-longer-slowing-down-earth.html |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" />

==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>
|-
! Year !! 30 Jun !! 31 Dec
|-
! 1972
|bgcolor="lime"| +1 ||bgcolor="lime"| +1
|-
! 1973
| 0 ||bgcolor="lime"| +1
|-
! 1974
| 0 ||bgcolor="lime"| +1
|-
! 1975
| 0 ||bgcolor="lime"| +1
|-
! 1976
| 0 ||bgcolor="lime"| +1
|-
! 1977
| 0 ||bgcolor="lime"| +1
|-
! 1978
| 0 ||bgcolor="lime"| +1
|-
! 1979
| 0 ||bgcolor="lime"| +1
|-
! 1980
| 0 || 0
|-
! 1981
|bgcolor="lime"| +1 || 0
|-
! 1982
|bgcolor="lime"| +1 || 0
|-
! 1983
|bgcolor="lime"| +1 || 0
|-
! 1984
| 0 || 0
|-
! 1985
|bgcolor="lime"| +1 || 0
|-
! 1986
| 0 || 0
|-
! 1987
| 0 ||bgcolor="lime"| +1
|-
! 1988
| 0 || 0
|-
! 1989
| 0 ||bgcolor="lime"| +1
|-
! 1990
| 0 ||bgcolor="lime"| +1
|-
! 1991
| 0 || 0
|-
! 1992
|bgcolor="lime"| +1 || 0
|-
! 1993
|bgcolor="lime"| +1 || 0
|-
! 1994
|bgcolor="lime"| +1 || 0
|-
! 1995
| 0 ||bgcolor="lime"| +1
|-
! 1996
| 0 || 0
|-
! 1997
|bgcolor="lime"| +1 || 0
|-
! 1998
| 0 ||bgcolor="lime"| +1
|-
! 1999
| 0 || 0
|-
! 2000
| 0 || 0
|-
! 2001
| 0 || 0
|-
! 2002
| 0 || 0
|-
! 2003
| 0 || 0
|-
! 2004
| 0 || 0
|-
! 2005
| 0 ||bgcolor="lime"| +1
|-
! 2006
| 0 || 0
|-
! 2007
| 0 || 0
|-
! 2008
| 0 ||bgcolor="lime"| +1
|-
! 2009
| 0 || 0
|-
! 2010
| 0 || 0
|-
! 2011
| 0 || 0
|-
! 2012
|bgcolor="lime"| +1 || 0
|-
! 2013
| 0 || 0
|-
! 2014
| 0 || 0
|-
! 2015
|bgcolor="lime"| +1 || 0
|-
! 2016
| 0 ||bgcolor="lime"| +1
|-
! 2017
| 0 || 0
|-
! 2018
| 0 || 0
|-
! 2019
| 0 || 0
|-
! 2020
| 0 || 0
|-
! 2021
| 0 || 0
|-
! 2022
| 0 || 0
|-
! 2023
| 0 || 0
|-
! 2024
| 0 || 0
|-
! 2025
| 0 || 0
|-
! Year !! 30 Jun !! 31 Dec
|-
!rowspan="2"| Total
| 11 || 16
|-
|colspan="2"| 27
|-
!colspan="3"| Current TAI − UTC
|-
|colspan="3"| 37
|}

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&nbsp;s, in order to keep the difference between UTC and UT1 from exceeding 0.9&nbsp;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 its "Bulletin C".<ref>{{cite web |url=http://hpiers.obspm.fr/iers/bul/bulc/bulletinc.dat | title=Bulletin C}}</ref>   Such announcements are typically published well in advance of each possible leap second date – usually in early January for 30 June and in early July for 31 December.<ref name="Bulletin C 36">{{cite web|last=Gambis|first=Daniel|title=Bulletin C 36|publisher=[[International Earth Rotation and Reference Systems Service|IERS EOP PC]], [[Observatoire de Paris]]|location=Paris|date=4 July 2008|url=http://tycho.usno.navy.mil/bulletinc2008.html|access-date=18 April 2010|url-status=dead|archive-url=https://web.archive.org/web/20091006221408/http://tycho.usno.navy.mil/bulletinc2008.html|archive-date=6 October 2009}}</ref><ref>{{Cite web|url=http://www.livescience.com/strangenews/081208-leap-second.html|title=2008 Will Be Just a Second Longer|author=Andrea Thompson|access-date=29 December 2008|publisher=Live Science|date=8 December 2008|url-status=live|archive-url=https://web.archive.org/web/20081212222023/http://www.livescience.com/strangenews/081208-leap-second.html|archive-date=12 December 2008}}</ref> Some [[time signal]] broadcasts give voice announcements of an impending leap second.
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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 2004, but there were nine leap seconds in the eight years 1972–1979.  Since the introduction of leap seconds, 1972 has been the longest year on record: 366 days, 364 of which were 86,400 seconds long and two of which were 86,401 seconds long, for a total of 31,622,402 seconds.

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==
{{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 [[IERS Reference Meridian]] at Greenwich). UTC (on which [[civil time]] is usually based) is a compromise, stepping with atomic seconds but periodically reset by a leap second to match UT1.

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>

===Elimination proposals===
{{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=https://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.

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=dead |archive-url=https://web.archive.org/web/20170208050335/http://tycho.usno.navy.mil/papers/ts-2014/Matsakis-LeapSecondComments.URSI-2014.pdf |archive-date=8 February 2017}}</ref> He stressed the practical inability of software programmers to allow for the fact that leap seconds make time appear to go backwards, particularly when most of them do not even know that leap seconds exist. The possibility of leap seconds being a hazard to navigation was presented, as well as the observed effects on commerce.

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>}}

In 2011, Chunhao Han of the [[Beijing Global Information Center of Application and Exploration]] said China had not decided what its vote would be in January 2012, but some Chinese scholars consider it important to maintain a link between civil and astronomical time due to Chinese tradition. The 2012 vote was ultimately deferred.<ref name=merali2011>{{cite journal|last=Merali|first=Zeeya|date=8 November 2011|title=Time is running out for the leap second|journal=Nature|volume=479 |issue=7372 |page=158 |doi=10.1038/479158a |pmid=22071738 |bibcode=2011Natur.479..158M |s2cid=8220495 |doi-access=free }}</ref> At an ITU/BIPM-sponsored workshop on the leap second, Han expressed his personal view in favor of abolishing the leap second,<ref>{{cite web|url=https://www.itu.int/dms_pub/itu-r/oth/0a/0e/R0A0E0000960001PDFE.pdf|title=Conception, Definition and Realization of Time Scale in GNSS|last=Han|first=Chunhao|date=19 September 2013|url-status=live|archive-url=https://web.archive.org/web/20140905221335/https://www.itu.int/dms_pub/itu-r/oth/0a/0e/R0A0E0000960001PDFE.pdf|archive-date=5 September 2014}}</ref> and similar support for the redefinition was again expressed by Han, along with other Chinese timekeeping scientists, at the URSI General Assembly in 2014.

At a special session of the [[Asia-Pacific Telecommunity]] meeting on 10 February 2015, Chunhao Han indicated China was now supporting the elimination of future leap seconds, as were all the other presenting national representatives (from Australia, Japan, and the Republic of Korea).  At this meeting, Bruce Warrington (NMI, Australia) and Tsukasa Iwama (NICT, Japan) indicated particular concern for the financial markets due to the leap second occurring in the middle of a workday in their part of the world.{{efn|In addition to publishing the video of the special session,<ref>{{cite AV media|url=https://www.youtube.com/watch?v=gsyFp6G6iKo|title=Information Session on the WRC-15 agenda item 1.14 – Coordinated Universal Time (UTC)|date=15 April 2015|work=YouTube|url-status=live|archive-url=https://web.archive.org/web/20151118220255/https://www.youtube.com/watch?v=gsyFp6G6iKo|archive-date=18 November 2015}}</ref> the Australian Communications and Media Authority has a transcript of that session and a web page with draft content of the Conference Preparatory Meeting report and solutions for ITU-R WRC-15 Agenda Item 1.14.<ref>{{cite web|url=http://www.acma.gov.au/Industry/Spectrum/Spectrum-planning/International-planning-ITU-and-other-international-planning-bodies/wrc-15-agenda-item-114|archive-url=https://web.archive.org/web/20150908184107/http://www.acma.gov.au/Industry/Spectrum/Spectrum-planning/International-planning-ITU-and-other-international-planning-bodies/wrc-15-agenda-item-114|archive-date= 8 September 2015|url-status=dead|title=WRC-15 Agenda item 1.14: Coordinated Universal Time (UTC)|work=acma.gov.au}}</ref>}}  Subsequent to the CPM15-2 meeting in March/April 2015 the draft gives four methods which the WRC-15 might use to satisfy Resolution 653 from WRC-12.<ref>{{cite web|url=http://www.itu.int/dms_pub/itu-r/oth/0c/0a/R0C0A00000A0022PDFE.pdf|title=RESOLUTION 653 (WRC-12) Future of the Coordinated Universal Time time-scale|publisher=[[International Telecommunication Union]]|url-status=live|archive-url=https://web.archive.org/web/20150702175848/http://www.itu.int/dms_pub/itu-r/oth/0c/0a/R0C0A00000A0022PDFE.pdf|archive-date=2 July 2015}}</ref>

Arguments against the proposal include the unknown expense of such a major change and the fact that universal time will no longer correspond to mean solar time. It is also answered that two timescales that do not follow leap seconds are already available, [[International Atomic Time]] (<span title='Temps Atomique International' lang='fr'>TAI</span>) and [[Global Positioning System]] (GPS) time. Computers, for example, could use these and convert to UTC or local civil time as necessary for output. Inexpensive GPS timing receivers are readily available, and the satellite broadcasts include the necessary information to convert [[GPS time]] to UTC. It is also easy to convert GPS time to TAI, as TAI is always exactly 19 seconds ahead of GPS time. Examples of systems based on GPS time include the [[CDMA]] digital cellular systems [[IS-95]] and [[CDMA2000]]. In general, computer systems use UTC and synchronize their clocks using [[Network Time Protocol]] (NTP). Systems that cannot tolerate disruptions caused by leap seconds can base their time on TAI and use [[Precision Time Protocol]]. However, the BIPM has pointed out that this proliferation of timescales leads to confusion.<ref>{{citation|url=https://www.bipm.org/documents/20126/2071143/CCTF+Strategy.pdf/7cf0f648-2afe-d15c-0909-1f03406bbb8f|date=May 2016|title=CCTF Strategy Document|publisher=[[International Bureau of Weights and Measures]]|pages=21–25}}</ref>

At the 47th meeting of the Civil Global Positioning System Service Interface Committee in [[Fort Worth]], [[Texas]], in September 2007, it was announced that a mailed vote would go out on stopping leap seconds. The plan for the vote was:<ref>{{cite web|url=http://www.navcen.uscg.gov/pdf/cgsicMeetings/47/%5B16%5D%20CGSIC47-WL%20General_md.pdf|access-date=18 November 2007|date=25 September 2007|pages=9|title=47th CGSIC Meeting – Timing Subcommittee|url-status=dead|archive-url=https://web.archive.org/web/20110614015830/http://www.navcen.uscg.gov/pdf/cgsicMeetings/47/%5B16%5D%20CGSIC47-WL%20General_md.pdf|archive-date=14 June 2011}}</ref>
* April 2008: [[ITU]] Working Party 7A will submit to ITU Study Group 7 project recommendation on stopping leap seconds
* During 2008, Study Group 7 will conduct a vote through mail among member states
* October 2011: The ITU-R released its status paper, ''Status of Coordinated Universal Time (UTC) study in ITU-R'', in preparation for the January 2012 meeting in Geneva; the paper reported that, to date, in response to the UN agency's 2010 and 2011 web-based surveys requesting input on the topic, it had received 16 responses from the 192 Member States with "13 being in favor of change, 3 being contrary."<ref>{{cite journal|title=WP7D – Status of Coordinated Universal Time (UTC) study in ITU-R|journal=International Telecommunication Union – Radiocommunication Sector (ITU-R) Release|date=4 October 2011|page=2|url=http://www.itu.int/dms_pub/itu-r/oth/0A/08/R0A080000090001MSWE.docx|format=Word 2007|access-date=24 October 2011|quote=To date, the BR received replies from 16 different Member States for the latest survey (out of a total of 192 Member States, 55 of which participate in the formation of UTC) – 13 being in favor of the change, 3 being contrary.|url-status=live|archive-url=https://web.archive.org/web/20140323233615/http://www.itu.int/dms_pub/itu-r/oth/0A/08/R0A080000090001MSWE.docx|archive-date=23 March 2014}}</ref>
* January 2012: The ITU makes a decision.
In January 2012, rather than decide yes or no per this plan, the ITU decided to postpone a decision on leap seconds to the [[World Radiocommunication Conference]] in November 2015. At this conference, it was again decided to continue using leap seconds, pending further study and consideration at the next conference in 2023.<ref name=WRC>{{cite press release| title = Coordinated Universal Time (UTC) to retain 'leap second' | publisher = International Telecommunication Union | date = 19 November 2015| url = https://www.itu.int/net/pressoffice/press_releases/2015/53.aspx | url-status=dead<!--might be temporary? 18Nov22 (and 26May23)-->| archive-url = https://web.archive.org/web/20160129232825/https://www.itu.int/net/pressoffice/press_releases/2015/53.aspx | archive-date = 29 January 2016}}</ref>

In October 2014, Włodzimierz Lewandowski, chair of the timing subcommittee of the Civil GPS Interface Service Committee and a member of the ESA Navigation Program Board, presented a CGSIC-endorsed resolution to the ITU that supported the redefinition and described leap seconds as a "hazard to navigation".<ref>{{cite web|url=http://www.gps.gov/cgsic/timing/2014-resolution/|title=CGSIC opinion on the redefinition of UTC now under consideration by the International Telecommunication Union (ITU)|url-status=live|archive-url=https://web.archive.org/web/20141023113730/http://www.gps.gov/cgsic/timing/2014-resolution/|archive-date=23 October 2014}}</ref>

Some of the objections to the proposed change have been addressed by its supporters. For example, [[Felicitas Arias]], who, as Director of the [[International Bureau of Weights and Measures]] (BIPM)'s Time, Frequency, and Gravimetry Department, was responsible for generating UTC, noted in a press release that the drift of about one minute every 60–90 years could be compared to the 16-minute annual variation between true solar time and mean solar time, the one hour offset by use of daylight time, and the several-hours offset in certain geographically extra-large time zones.<ref>{{cite press release|url=http://www.bipm.org/utils/en/pdf/Press_Release_UTC_13October.pdf|title=The proposed redefinition of Coordinated Universal Time, UTC|publisher=[[BIPM]]|date=13 October 2011|url-status=live|archive-url=https://web.archive.org/web/20150118133939/http://www.bipm.org/utils/en/pdf/Press_Release_UTC_13October.pdf|archive-date=18 January 2015}}</ref>

Proposed alternatives to the leap second are the leap hour, which requires changes only once every few centuries;<ref>{{cite magazine |title=Scientists propose 'leap hour' to fix time system |url=https://www.newindianexpress.com/magazine/2008/dec/18/scientists-propose-leap-hour-to-fix-time-system-11669.html |magazine=New Scientist |via=The New Indian Express |access-date=3 September 2022 |orig-date=2008-12-18 |date=14 May 2012}}</ref> and the leap minute, with changes coming every half-century.<ref name="Martin-2024" /><ref>{{Cite news| issn = 0362-4331| last = Richtel| first = Matt| title = A Giant Leap for the Leap Second. Is Humankind Ready?| work = The New York Times| access-date = 23 January 2024| date = 3 November 2023| url = https://www.nytimes.com/2023/11/03/science/time-leap-second.html}}</ref>

On 18 November 2022, the [[General Conference on Weights and Measures]] (CGPM) resolved to eliminate leap seconds by or before 2035. The difference between atomic and astronomical time will be allowed to grow to a larger value yet to be determined. A suggested possible future measure would be to let the discrepancy increase to a full minute, which would take 50 to 100&nbsp;years, and then have the last minute of the day taking two minutes in a "kind of smear" with no discontinuity. The year 2035 for eliminating leap seconds was chosen considering Russia's request to extend the timeline to 2040, since, unlike the United States's [[global navigation satellite system]], [[GPS]], which does not adjust its time with leap seconds, Russia's system, [[GLONASS]], does adjust its time with leap seconds.<ref name="AFP">{{cite news |first= |date=18 November 2022 |title=Do not adjust your clock: scientists call time on the leap second |url=https://www.theguardian.com/world/2022/nov/18/do-not-adjust-your-clock-scientists-call-time-on-the-leap-second |website=[[The Guardian]] |department=World News |agency=[[Agence France-Presse]]}}</ref><ref name="gibney">{{cite journal |last=Gibney |first=Elizabeth |author-link=Elizabeth Gibney |date=18 November 2022 |title=The leap second's time is up: world votes to stop pausing clocks |journal=[[Nature (journal)|Nature]] |volume=612 |issue=7938 |page=18 |bibcode=2022Natur.612...18G |doi=10.1038/d41586-022-03783-5 |issn=0028-0836 |pmid=36400956 |doi-access=free}}</ref>

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.]]
===Time difference and sequence calculations===
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 announcements===
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]] (NTP) freezes the count of time during the leap second,<ref>{{Cite web |date=10 February 2010 |title=NIST Internet Time Service (ITS) |url=https://www.nist.gov/pml/time-and-frequency-division/time-distribution/internet-time-service-its |website=NIST}}</ref> some time servers declare "alarm condition".{{citation needed|date=October 2019}} Other schemes ''smear'' time in the vicinity of a leap second, spreading out the second of change over a longer period. This aims to avoid any negative effects of a substantial (by modern standards) step in time.<ref>{{citation|rfc=7164|author=Kevin Gross|title=RTP and Leap Seconds|date=March 2014}}</ref><ref name=google-smear /> This approach has led to differences between systems, as leap smear is not standardized and several different schemes are used in practice.<ref>{{cite web | website = Bureau International des Poids et Mesures | url = https://www.bipm.org/documents/20126/77823803/INTRODUCTION-DRAFT-RESOLUTION-D.pdf/62f31b32-bfe4-969f-5d29-1dc08cc907bb?version=1.2&t=1669373890355&download=true | title = Draft Resolution D: 'On the use and future development of UTC' | last1 = Dimarcq | first1 = Noël | last2 = Tavella | first2 = Patrizia  | date = 17 November 2022 | page = 7}}</ref>

===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 TymServe 2100 GPS receivers apply a leap second as soon as the a leap second notification is received, instead of waiting for the correct date. The manufacturers no longer supports these models and no corrected software is available. A workaround has been described and tested, but if the GPS system rebroadcasts the announcement, or the unit is powered off, the problem will occur again.<ref>{{cite mailing list|url=http://permalink.gmane.org/gmane.comp.time.nuts/43942|title=Symmetricom TymServe 2100-GPS currently fails with GPS offset|mailing-list=time-nuts|url-status=live|archive-url=https://web.archive.org/web/20150217202809/http://permalink.gmane.org/gmane.comp.time.nuts/43942|archive-date=17 February 2015}}</ref>
* 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.

Several software vendors have distributed software that has not properly functioned with the concept of leap seconds:

* NTP specifies a flag to inform the receiver that a leap second is imminent. However, some NTP server implementations have failed to set their leap second flag correctly.<ref>{{cite web |url=http://www.maths.tcd.ie/~dwmalone/time/leaps/ |title=NTP Leap Bits |first1=David |last1=Malone |access-date=1 December 2019 |url-status=live |archive-url=https://web.archive.org/web/20140722122446/http://www.maths.tcd.ie/~dwmalone/time/leaps/ |archive-date=22 July 2014}}</ref><ref>{{cite conference |first1=David |last1=Malone |year=2016 |title=The Leap Second Behaviour of NTP Servers |book-title=Proc.Traffic Monitoring and Analysis workshop |url=http://tma.ifip.org/2016/papers/tma2016-final27.pdf |access-date=23 October 2016 |url-status=live |archive-url=https://web.archive.org/web/20161023201948/http://tma.ifip.org/2016/papers/tma2016-final27.pdf |archive-date=23 October 2016}}</ref><ref name="sync_Leap2016">{{cite conference |last1=Cao |first1=Yi |last2=Veitch |first2=Darryl |title=Network Timing, Weathering the 2016 Leap Second |doi=10.1109/INFOCOM.2018.8486286 |pages=1826–1834 |conference=IEEE Infocom 2018 |location=Honolulu, Hawaii |date= 15–19 April 2018|hdl=10453/130538 |hdl-access=free }}</ref><ref name="sync_Leap2015">{{cite conference |last1=Veitch |first1=Darryl |last2=Vijayalayan |first2=Kanthaiah |title=Network Timing and the 2015 Leap Second |doi=10.1007/978-3-319-30505-9_29 |pages=385–396 |book-title=Proc. of PAM 2016 |location=Heraklion, Crete, Greece |date=1 April 2016|hdl=10453/43923 |hdl-access=free }}</ref> Some NTP servers have responded with the wrong time for up to a day after a leap second insertion.<ref>{{cite web|url=http://www.satsignal.eu/ntp/ntp-events.htm|title=NTP Events|work=satsignal.eu|url-status=live|archive-url=https://web.archive.org/web/20140718204646/http://www.satsignal.eu/ntp/ntp-events.htm|archive-date=18 July 2014}}</ref>
* A number of organizations reported problems caused by flawed software following the leap second that occurred on 30 June 2012. Among the sites which reported problems were [[Reddit]] ([[Apache Cassandra]]), [[Mozilla]] ([[Hadoop]]),<ref>{{cite magazine |date=1 July 2012 |title='Leap Second' Bug Wreaks Havoc Across Web |url=https://www.wired.com/wiredenterprise/2012/07/leap-second-bug-wreaks-havoc-with-java-linux/ |url-status=live |magazine=Wired |archive-url=https://web.archive.org/web/20140328124116/http://www.wired.com/wiredenterprise/2012/07/leap-second-bug-wreaks-havoc-with-java-linux |archive-date=28 March 2014}}</ref> [[Qantas]],<ref>{{cite web |date=1 July 2012 |title=Leap second crashes Qantas and leaves passengers stranded |url=http://www.news.com.au/travel/news/leap-second-crashes-qantas-and-leaves-passengers-stranded/story-e6frfq80-1226413961235 |url-status=live |archive-url=https://web.archive.org/web/20120701162924/http://www.news.com.au/travel/news/leap-second-crashes-qantas-and-leaves-passengers-stranded/story-e6frfq80-1226413961235 |archive-date=1 July 2012 |publisher=News Limited}}</ref> and various sites running Linux.<ref>{{cite web |title=Anyone else experiencing high rates of Linux server crashes during a leap second day? |url=http://serverfault.com/questions/403732/anyone-else-experiencing-high-rates-of-linux-server-crashes-during-a-leap-second |url-status=live |archive-url=https://web.archive.org/web/20120709110353/http://serverfault.com/questions/403732/anyone-else-experiencing-high-rates-of-linux-server-crashes-during-a-leap-second |archive-date=9 July 2012 |publisher=Serverfault.com}}</ref>
* Despite the publicity given to the 2015 leap second, a small number of network failures occurred due to leap second-related software errors of some routers.<ref>{{cite news |last1=Shulman |first1=Eden |title=Beta Boston |newspaper=Boston Globe |url=http://www.betaboston.com/news/2015/07/02/leap-second-confuses-some-internet-routers-bringing-down-a-small-slice-of-networks/ |url-status=live |access-date=27 September 2015 |archive-url=https://web.archive.org/web/20150929011652/http://www.betaboston.com/news/2015/07/02/leap-second-confuses-some-internet-routers-bringing-down-a-small-slice-of-networks/ |archive-date=29 September 2015}}</ref>  Several older versions of the [[Cisco Systems]] Nexus 5000 Series Operating System NX-OS (versions 5.0, 5.1, 5.2) are affected by leap second bugs.<ref>{{cite web |date=24 July 2015 |title=Cisco Bug: CSCub38654 – N5K: Switch hang/lock up may occur due to Leap second update |url=https://quickview.cloudapps.cisco.com/quickview/bug/CSCub38654 |url-status=live |archive-url=https://web.archive.org/web/20160308010403/https://quickview.cloudapps.cisco.com/quickview/bug/CSCub38654 |archive-date=8 March 2016 |publisher=Cisco}}</ref>

Some businesses and service providers have been impacted by leap-second related software bugs:

* In 2015, interruptions occurred with [[Twitter]], [[Instagram]], [[Pinterest]], [[Netflix]], [[Amazon (company)|Amazon]], and [[Apple Inc|Apple's]] music streaming series [[Beats 1]].<ref>{{cite news |author=Sarah Knapton |date=1 July 2015 |title=Leap Second confuses Twitter and Android |newspaper=[[The Daily Telegraph]] |url=https://www.telegraph.co.uk/news/science/science-news/11710148/Leap-Second-confuses-Twitter-and-Android.html |url-status=live |archive-url=https://web.archive.org/web/20151006002636/http://www.telegraph.co.uk/news/science/science-news/11710148/Leap-Second-confuses-Twitter-and-Android.html |archive-date=6 October 2015}}</ref>
* Leap second software bugs in Linux reportedly affected the [[Amadeus Altéa]] airlines reservation system, used by Qantas and [[Virgin Australia]], in 2015.<ref>{{Cite web |last=Clarke |first=Gavin |date=8 August 2016 |title=Power cut crashes Delta's worldwide flight update systems |url=https://www.theregister.co.uk/2016/08/08/computer_fault_takes_down_delta/ |url-status=live |archive-url=https://web.archive.org/web/20170104002416/http://www.theregister.co.uk/2016/08/08/computer_fault_takes_down_delta/ |archive-date=4 January 2017 |access-date=3 January 2017 |website=[[The Register]]}}</ref>
* [[Cloudflare]] was affected by a leap second software bug. Its [[DNS]] resolver implementation incorrectly calculated a negative number when subtracting two timestamps obtained from the [[Go programming language]]'s <code>time.Now()</code>function, which then used only a [[real-time clock]] source.<ref>{{cite web |date=1 January 2017 |title=How and why the leap second affected Cloudflare DNS |url=https://blog.cloudflare.com/how-and-why-the-leap-second-affected-cloudflare-dns/ |url-status=live |archive-url=https://web.archive.org/web/20170102112428/https://blog.cloudflare.com/how-and-why-the-leap-second-affected-cloudflare-dns/ |archive-date=2 January 2017 |publisher=Cloudflare}}</ref>  This could have been avoided by using a monotonic clock source, which has since been added to Go 1.9.<ref>{{cite web |title=#12914 runtime: time: expose monotonic clock source |url=https://github.com/golang/go/issues/12914 |url-status=live |archive-url=https://web.archive.org/web/20170320012113/https://github.com/golang/go/issues/12914 |archive-date=20 March 2017 |access-date=5 January 2017 |website=GitHub}}</ref>
* The [[Intercontinental Exchange]], parent body to 7 clearing houses and 11 stock exchanges including the [[New York Stock Exchange]], chose to cease operations for 61 minutes at the time of the 30 June 2015, leap second.<ref>{{cite web|url=https://www.theice.com/leap-second|title=ICE Market Update – Leap Second Impact|publisher=[[Intercontinental Exchange]]|url-status=dead|archive-url=https://web.archive.org/web/20150505061854/https://www.theice.com/leap-second|archive-date=5 May 2015}}</ref>

There were misplaced concerns that farming equipment using GPS navigation during harvests occurring on 31 December 2016, would be affected by the 2016 leap second.<ref>{{Cite news|url=http://www.abc.net.au/news/2016-12-30/2016-leap-second-correction-for-gps-farmers/8154156|title=Got a second – the world time needs it|date=30 December 2016|newspaper=ABC Rural|language=en-AU|access-date=3 January 2017|url-status=live|archive-url=https://web.archive.org/web/20170102192801/http://www.abc.net.au/news/2016-12-30/2016-leap-second-correction-for-gps-farmers/8154156|archive-date=2 January 2017}}</ref> GPS navigation makes use of [[GPS time]], which is not impacted by the leap second.<ref>{{citation|url=https://www.scientificamerican.com/article/how-fast-is-the-earth-mov/|date=26 October 1998|title=How fast is the earth moving? Rhett Herman, a physics professor at Radford University in Virginia, supplies the following answer|publisher=[[Scientific American]]}}</ref>

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>

Instead of inserting a leap second at the end of the day, [[Google]] servers implement a "leap smear", extending seconds slightly over a 24-hour period centered on the leap second.<ref name=google-smear>{{cite web|url=https://developers.google.com/time/smear|title=Leap Smear|publisher=Google Inc.|access-date=26 May 2023}}</ref>  Amazon followed a similar, but slightly different, pattern for the introduction of the 30 June 2015 leap second,<ref>{{cite web|url=https://aws.amazon.com/blogs/aws/look-before-you-leap-the-coming-leap-second-and-aws/|title=Look Before You Leap – The Coming Leap Second and AWS (Updated)|author1=Jeff Barr|date=18 May 2015|publisher=[[Amazon Web Services]]}}</ref> leading to another case of the proliferation of timescales. They later released an NTP service for [[Amazon Elastic Compute Cloud|EC2]] instances which performs leap smearing.<ref>{{cite web|url=https://aws.amazon.com/blogs/aws/keeping-time-with-amazon-time-sync-service/|title=Keeping Time With Amazon Time Sync Service|author1=Randall Hunt|date=29 November 2017|publisher=[[Amazon Web Services]]|access-date=8 March 2018}}</ref> UTC-SLS was proposed as a version of UTC with linear leap smearing, but it never became standard.<ref>{{cite web |last1=Kuhn |first1=Markus |title=UTC with Smoothed Leap Seconds (UTC-SLS) |url=https://www.cl.cam.ac.uk/~mgk25/time/utc-sls/|date=2005 |website=www.cl.cam.ac.uk}}</ref>

It has been proposed that media clients using the [[Real-time Transport Protocol]] inhibit generation or use of NTP timestamps during the leap second and the second preceding it.<ref>{{cite ietf|rfc=7164|title=RTP and Leap Seconds|author=Kevin Gross|date=March 2014}}</ref>

NIST has established a special NTP time server to deliver UT1 instead of UTC.<ref>{{cite web |url=https://www.nist.gov/pml/time-and-frequency-division/time-services/ut1-ntp-time-dissemination |title=UT1 NTP Time Dissemination |website=[[National Institute of Standards and Technology]] |date=11 December 2015 |access-date=31 August 2019}}</ref>  Such a server would be particularly useful in the event the ITU resolution passes and leap seconds are no longer inserted.<ref>{{cite conference|first1=Patrick|last1=Wallace|year=2003|title=The UTC Problem and its Solution|book-title=Proceedings of Colloquium on the UTC Time Scale|location=Torino|url=http://www.ucolick.org/~sla/leapsecs/torino/wallace.pdf|url-status=live|archive-url=https://web.archive.org/web/20150118134330/http://www.ucolick.org/~sla/leapsecs/torino/wallace.pdf|archive-date=18 January 2015}}</ref> Those astronomical observatories and other users that require UT1 could run off UT1 – although in many cases these users already download UT1-UTC from the IERS, and apply corrections in software.<ref>{{cite conference|first1=Brian|last1=Luzum|title=The Role of the IERS in the Leap Second|conference=BIPM/ITU Workshop on the Future of the International Time Scale|year=2013|url=https://www.itu.int/dms_pub/itu-r/oth/0a/0e/R0A0E0000960016PDFE.pdf|url-status=live|archive-url=https://web.archive.org/web/20140715055118/https://www.itu.int/dms_pub/itu-r/oth/0a/0e/R0A0E0000960016PDFE.pdf|archive-date=15 July 2014}}</ref>

==See also==
* [[Clock drift]], phenomenon where a clock gains or loses time compared to another clock
* [[DUT1]], which describes the difference between coordinated universal time (UTC) and universal time (UT1)
* [[Dynamical time scale]]
* [[Leap year]], a year containing one extra day or month

==Notes==
{{notelist}}

==References==
{{reflist}}

==Further reading==
* {{Cite news |author-link=Anjana Ahuja |last=Ahuja |first=Anjana |date=30 October 2005 |title=Savouring the last leap second in history |newspaper=[[New Straits Times]] |page=F10}}
* {{Cite magazine |last=Grossman |first=Wendy M. |date=1 November 2005 |title=Wait a Second |url=https://www.scientificamerican.com/article/wait-a-second/ |magazine=[[Scientific American]] |volume=293 |issue=5 |pages=12–13 |doi=10.1038/scientificamerican1105-24}}
* {{cite journal | last1=Finkleman | first1=David | last2=Allen | first2=Steve | last3=Seago | first3=John | last4=Seaman | first4=Rob | last5=Seidelmann | first5=P. Kenneth | title=The Future of Time: UTC and the Leap Second | year=2011 | url = https://www.americanscientist.org/article/the-future-of-time-utc-and-the-leap-second | arxiv = 1106.3141 | journal = [[American Scientist]] | volume = 99 | issue = 4 | pages = 312–319 | doi = 10.1511/2011.91.312 | s2cid = 118403321}}
* {{cite journal | last1 = Kamp | first1 = Poul-Henning | year = 2011 | title = The One-Second War | journal = [[Communications of the ACM]] | volume = 54 | issue = 5| pages = 44–48 | doi = 10.1145/1941487.1941505 | doi-access = free }}
* {{Cite book |author-link=Dennis McCarthy (scientist) |last1=McCarthy |first1=Dennis D. |last2=Seidelmann |first2=P. Kenneth |year=2009 |title=TIME From Earth Rotation to Atomic Physics |place=[[Weinheim]] |doi=10.1002/9783527627943 |publisher=[[Wiley-VCH]]|isbn=978-3527407804 }}

==External links==
{{commons category|Leap second}}
*[https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html IERS Bulletins, including Bulletin C (leap second announcements)]
*[http://www.leapsecond.com/ LeapSecond.com – A web site dedicated to precise time and frequency]
*[https://www.nist.gov/pml/time-and-frequency-division/leap-seconds-faqs NIST FAQ about leap year and leap second]
*[https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf The leap second: its history and possible future]
*{{cite web|url=https://support.microsoft.com/help/2722715/support-for-the-leap-second|title=Support for the leap second|website=Microsoft Support|date=4 October 2018}}
*{{cite web|url=https://techcommunity.microsoft.com/t5/Networking-Blog/Leap-Seconds-for-the-IT-Pro-What-you-need-to-know/ba-p/339811|title=Leap Seconds for the IT Pro: What you need to know|author=Dan Cuomo|website=Windows Server – Networking Blog|date=17 October 2018}}
*{{cite web|url=https://techcommunity.microsoft.com/t5/Networking-Blog/Leap-Seconds-for-the-AppDev-What-you-should-know/ba-p/339813|title=Leap Seconds for the AppDev: What you should know|author=Travis Luke|website=Windows Server – Networking Blog|date=24 October 2018}}
*{{cite web|url=https://safran-navigation-timing.com/leap-second-readiness-tips-a-guide-for-managers-of-critical-it-infrastructure/|title=Leap Second Readiness Tips|publisher=Safran (formerly Orolia)|website=safran-navigation-timing.com|date=31 December 2018}}
* Judah Levine's Everyday Time and Atomic Time series
**{{cite web|url=https://nist.medium.com/everyday-time-and-atomic-time-part-one-9107b60fd9d0
|author=Judah Levine
|title=Everyday Time and Atomic Time: Part One
|publisher=National Institute of Standards and Technology
|date=31 March 2021}}
**{{cite web|url=https://nist.medium.com/everyday-time-and-atomic-time-part-2-65bb74d04f50
|author=Judah Levine
|title=Everyday Time and Atomic Time: Part Two
|publisher=National Institute of Standards and Technology
|date=7 April 2021}}
**{{cite web|url=https://nist.medium.com/everyday-time-and-atomic-time-part-3-24421c7a8c0f
|author=Judah Levine
|title=Everyday Time and Atomic Time: Part Three
|publisher=National Institute of Standards and Technology
|date=14 April 2021}}
**{{cite web|url=https://nist.medium.com/everyday-time-and-atomic-time-part-4-5d9b27820851
|author=Judah Levine
|title=Everyday Time and Atomic Time: Part Four
|publisher=National Institute of Standards and Technology
|date=21 April 2021}}
**{{cite web|url=https://nist.medium.com/everyday-time-and-atomic-time-part-5-1a9f063a8b1b
|author=Judah Levine
|title=Everyday Time and Atomic Time: Part Five
|publisher=National Institute of Standards and Technology
|date=28 April 2021}}
{{Time Topics}}
{{Time measurement and standards}}
{{New Year}}

{{DEFAULTSORT:Leap Second}}
[[Category:Timekeeping]]
[[Category:1972 introductions]]
[[Category:1972 in science]]