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[[File:Leapsecond2016.png|thumb|right|300px|[[Screenshot]] of the [[UTC]] clock from {{URL|https://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 |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, one has not happened yet.
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