Second Leap

I'm telling you this now so you'll be ready on June 30th. On that day, we will add a second to the official time as it is kept at the International Earth Rotation and Reference Systems Service (IERS) in Paris, France.

The IERS uses an atomic clock based on the rate of vibration of a cesium atom. That atom shimmies just more than 9.1 million times a second and so that atomic clock keeps precise time that never changes. Our dumb ol' Earth, on the other hand, isn't nearly as careful about how quickly it spins on its axis or revolves around the Sun and so the amount of time in a day isn't always the same.

Every now and again, the folks at IERS either add or subtract a second from the world's calendars and clocks in order to make the Earth's time match the atomic clock time. They last did it in 2008 and this year will do it between June 30 and July 1. Once your clock hits 12:59:59 11:59:59 on June 30, it will actually take it two seconds to go to 0:00:00 on July 1 instead of one second.

This also marks one of the few times that France manages to tell other countries what to do instead of surrendering.

(ETA: Clock reset thanks to Dustbury)