Leap Second

June 30 is one of the two times (the other being December 31) when the addition or subtraction of a second from our clock time is allowed to coordinate atomic and astronomical time. The determination to adjust is made by the International Earth Rotation Service of the International Bureau of Weights and Measures at Paris. A leap second is an intercalary, one-second adjustment that keeps broadcast standards for time of day close to mean solar time. Leap seconds are necessary to keep time standards synchronized with civil calendars, the basis of which is astronomical. The announcement to insert a leap second is given whenever the difference between UTC and UT1 approaches one-half second, to keep the difference between UTC and UT1 from exceeding ±0.9 s. After UTC 23:59:59, a positive leap second at 23:59:60 would be counted, before the clock indicates 00:00:00 of the next day. Negative leap seconds are also possible should the Earth's rotation becomes slightly faster; in that case, 23:59:58 would be followed by 00:00:00.