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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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