Leap second. Leap Second, is the one second added to Coordinated Universal Time( UTC) each year on the last day of June and/or December. This coordinates UTC time with astronomical time, whose reference is Greenwich Mean Time( GMT).
The addition of the leap second is initiated by the International EarthRotation Services (IERS) and is done whenever Greenwich meridian is crossed with a deviation of more than 0.9 sec.
Leap seconds were introduced in 1972 after astronomical time and TAI time had already diverged by more than 10 seconds since their introduction in 1958. Since 1972, a total of 32 leap seconds have been inserted.