LEAP SECOND

An occasional adjustment of one second, added to, or subtracted from, Coordinated Universal Time ( UTC ) to bring it into approximate synchronism with UT-1, which is the time scale based on the rotation of the Earth. ( 188 ) Note 1: Adjustments, when required, are made at the end of June 30, or preferably, December 31, Universal Time, so that UTC never deviates from UT-1 by more than 0.9 second. Note 2: The last minute of the day on which an adjustment is made has 61 or 59 seconds.