From Trivia no Izumi:
On January 1, 2006, an extra second will be added at 9:00 AM. Apparently it's to readjust the clocks since the 24 hour cycle that's used all over the world is slightly inaccurate enough to warrant adding the second so that we're in sync with the actual length of a day based on the earth's rotation. Apparently these adjustments have to be made every few years.
So at 8:59:59 AM, the next second will be 8:59:60 AM, then 9:00:00 AM.