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, or
UT1. Without such a correction, time reckoned by
Earth's rotation drifts away from
atomic time because of irregularities in the Earth's rate of rotation. Since this system of correction was implemented in 1972, 26 leap seconds have been inserted, the most recent on June 30, 2015 at 23:59:60 UTC.