The International Earth Rotation and Reference Systems Service has announced that a leap second will be added on December 31, 2008 at 23h 59m 60s, meaning that this year will be exactly one second longer. The last leap second occurred Dec 31, 2005; they are added due to fluctuations in the rotational speed of the earth. You can read all about leap seconds on Wikipedia."
http://hpiers.obspm.fr/iers/bul/bulc/bulletinc.dathttp://en.wikipedia.org/wiki/Leap_second