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==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.
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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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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==
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