Leap day comes around every four years (give or take a few), but just what is a leap second and why does it happen? This video explains the science behind our lost time.

In this clip, Chief Scientist for Time Services (and cool title-holder) Demetrios Matsakis of the U.S. Naval Observatory, poses an interesting problem: The Earth's rotation is infinitesimally slowing at a rate far too small for us to notice, but not for our increasingly precise clocks. "What do you do," he asks, "if the Earth slows down and you have so many seconds in a day, your clocks are giving you the right answer and the Earth is telling you the wrong answer?" The answer, as explained in the video, is the leap second

The clip is from The Atlantic Tech, and is part of a larger video piece from The Atlantic on where time comes from, which features a whole bunch of cool atomic clocks, and which you can watch here.