New Year's Day to be a second late

Scientists will insert a "leap second" onto the end of December 31, by tweaking their atomic clocks, the world's most precise timekeepers, so as to be consistent with the Earth's rotation.

Leap seconds are inserted because atomic clocks tend to go slightly ahead of the Earth's rotation, largely because the rotation of the Earth has been slowing down.

It will be the first extra second in seven years. The first leap second was added on June 30, 1972, according to NIST, an arm of the U.S. Commerce Department.

Above extracted from
http://news.xinhuanet.com/english/2005-12/26/content_3969840.htm