http://www.usatoday.com/tech/news/20...p-second_x.htm
The rotation of the Earth has been slowing down, so leap seconds keep the clocks and the Earth from getting out of synch with one another.
This will be the 23rd leap second that has been inserted since 1972 when an international timekeeping agreement was signed, according to the Observatory. The last one was inserted seven years ago.