A one-second correction that may be applied to atomic time to align it more closely with Universal Time, compensating for changes in the Earth's rotation. Since the first correction was made in 1972, all leap seconds have been positive, reflecting the general slowing of the Earth's rotation due to tidal braking. See also UTC. For more detailed information, including a list of all leap second corrections made to date, see http://tycho.usno.navy.mil/leapsec.html.
a second (see second, Systeme International) added between 60s and 0s at announced times to keep UTC within 0.90 s of UT1. Generally, leap seconds are added at the end of June or December.