A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to keep its time of day close to the mean solar time.
UTC definition specifies that the difference between UTC and the astronomically observed version of Universal Time, called UT1, must not exceed 0.9 seconds. It further recommends that the preferred time to insert the leap second is at 23h 59m 59s UTC on either 31 December or 30 June.
UTC leap seconds are irregularly spaced and unpredictable, due to the Earth’s rotation speed variation in response to climatic and geological events. Insertion of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service (IERS).
Since the UTC standard was established, negative leap seconds have never been needed. The most recent leap second was inserted on June 30, 2012 at 23:59:60 UTC.