A leap second has been scheduled for June 30, 2012. A leap second is an adjustment to the atomic clock-based Coordinated Universal Time (UTC) to bring it more closely in line with Universal Time (UT), which is based on the rotation of the earth. The two time standards do not agree because the earth’s rotation is ever so gradually and unpredictably slowing down.
I don't know about you, but "unpredictably slowing down" doesn't sound all that reassuring. But no far, I don't see any scientists and chronologists panicking. Link
(Image credit: Flickr user Alan Cleaver)