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 its "Bulletin C".<ref>See{{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.
