We're Getting an Extra Second on New Year's Eve

New Year's Eve will be just a little bit longer this year. The world's official clocks will have a "leap second" added to them on Dec. 31 at 23:59:59 Coordinated Universal Time (6:59:59pm Eastern time), Space.com reports. Leap seconds are periodically added in order to keep Earth's atomic clocks (which are based on "the natural vibrations of the cesium atom," Space notes) in sync with Earth's rotation (on which units of time were based before the atomic clock was introduced). The two "timescales" don't match up perfectly, so leap seconds must be added periodically to bring them back into sync. The last leap second was added on June 30, 2015; the leap second prior to that one brought down a chunk of the web. (More atomic clock stories.)