Today(June 30th 2012) Midnight clocks all over the world will get one extra second. This is known as Leap second. A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to keep its time of day close to the mean solar time.
Leap seconds are irregularly spaced because the Earth’s rotation speed changes irregularly. For example, there were no leap seconds in the 7 year time span between January 1, 1999 and December 31, 2005. But there were 9 leap seconds between 1972–1979.