From the NY Times:
The world’s timekeepers were preparing to adjust their clocks at midnight on Saturday to add an extra second to the day. … This will be the 25th since the system was introduced in 1972 … The adjustment is needed to reflect a slowing in the Earth’s rotation which gradually prolongs the solar day. The day has not been exactly 24 hours long since 1820. When dinosaurs roamed the planet, the day was only 23 hours long. … The leap second clicks in at 23:59:60 UTC (Coordinated Universal Time). In the U.S. that is 7:59:60 P.M. Eastern Daylight Time when the extra second is added to the atomic clock at the U.S. Naval Observatory’s Master Clock Facility in Washington.