This year – 2008 – is set to be the longest year in well over a decade. Not only did the fact that it's a leap year add an extra day, but now we're adding an extra second on 31 December.
The last time a year had an additional day plus a second was in 1992.
Because the Earth is constantly slowing down, the extra day in leap years isn't quite enough to keep atomic clocks synchronised with the planet's movement – hence the additional second.
The US Naval Oberservatory made the announcement yesterday, after an international group of timekeepers decided to implement the additional second.
Cell phones and computer operating systems will check with atomic clocks to adjust their times.