Every once in a while, the International Earth Rotation and Reference Systems Service (IERS) decides, after a carefull analysis, on adding an extra second to a certain year. This is exactly the case for 2008, which will receive its second at midnight. This is meant to help match people’s clocks to the Earth’s slowing spin on its axis, which occurs at ever-changing rates affected by tides and other factors.

The calculation scheme is made by the scientists at the IERS organization, which decides when it’s time for a new second; the extra seconds do not occur at an exact time, as they are known to have been added from six months to seven years of one another. The measurements of the Earth's rotation are gathered from all over the world and each January and July a notice is being issued in order to announce whether a leap second is required over the next six months.

The second is scheduled to be added at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time, also known as UTC. The plan is to make sure clocks vary from the Earth's rotational time by a maximum of 0.9 seconds before an adjustment. This keeps UTC in sync with the position of the sun above the Earth.

UTC is kept with the help of precise atomic clocks, built all over the world, accurate to about a billionth of a second per day.

These leap-seconds have been introduced in 1972 and the last one was added in 2005.