A "leap second" will be added to the world's official clocks on Dec. 31, 2016, to accommodate Earth's gradually slowing rotation. Revelers will get to celebrate New Year's Eve for a tiny bit longer than usual this year. A "leap second" will be added to the world's official clocks on Dec. 31 at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC), which corresponds to 6:59:59 p.m. EST; the clocks will read 23:59:60 before ticking over to midnight. The goal is to keep two different timescales in sync with each other. The units of time had long been defined based on Earth's rotation relative to distant celestial bodies. But that changed with the invention of atomic clocks in the mid-20th century; scientists then decided to base the second on the natural vibrations of the cesium atom. These two timescales don't match up exactly, however. Measurements show that, because the moon's gravitational pull and other factors are gradually slowing Earth's spin, the rotation-based