US Naval Observatory | 2016 July 06
On December 31, 2016, a “leap second” will be added to the world’s clocks at 23 hours, 59 minutes 59 seconds Coordinated Universal Time (UTC). This corresponds to 6:59:59 pm Eastern Standard Time, when the extra second will be inserted at the U.S. Naval Observatory’s Master Clock Facility in Washington, DC.
Don't forget to set your clocks back.