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{{short description|Extra second inserted to keep civil time in sync with the Earth's rotation}}
[[File:Leapsecond2016.png|thumb|right|300px|[[Screenshot]] of the [[UTC]] clock from {{URL|https://time.gov/}} during the leap second on 31 December 2016.]]▼
▲[[File:Leapsecond2016.png|thumb|right|300px|
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▼
▲A '''leap second''' is a one-
Because the Earth's rotational speed varies in response to climatic and geological events, 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.
This practice has proven disruptive, particularly in the twenty-first century and especially in services that depend on precise [[timestamp]]ing or time-critical [[process control]]. And since not all computers are adjusted by leap-second, they will display times differing from those that have been adjusted.<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 |arxiv=1106.3141 |doi=10.1511/2011.91.312 |s2cid=118403321 |doi-access=free}}</ref> After many years of discussions by different standards bodies, in November 2022, at the 27th [[General Conference on Weights and Measures]], it was decided to abandon the leap second by or before 2035.<ref name=AFP/><ref name=gibney/>▼
▲This practice has proven disruptive, particularly in the twenty-first century and especially in services that depend on precise
==History==
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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">{{Cite magazine |last=Martin |first=Cassie |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><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 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>
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