|On December 31, 2005, a leap second was added to the Coordinated Universal Time (atomic time) so that it would be in sync with Greenwich Mean Time (astronomical time).
This article talks about how certain scientists are proposing to get rid of the leap second because it could cause some major computer problems down the road. The reason the leap second needs to be added in the first place is because astronomical time is based on the movements of the Earth, and the Earth isn't perfect: Its days and years vary in length. Atomic time is based on the vibrations of the Cesium-133 atom, which doesn't change. So in order to keep the two methods of timekeeping in sync, the International Telecommunication Union decided that UTC couldn't differ from GMT by more than 9 tenths of a second. So every once in a while, a leap second is added so that everything stays in sync.
(via Boing Boing) #technology