"With a brutal economic slowdown, 2008 may feel as if it will never end. Now the world's timekeepers are making it even longer by adding a leap second to the last day of the year.
Along with the economy, the Earth itself is slowing down, requiring timekeepers to add an extra second to their atomic clocks to keep in sync with Earth's slightly slowing rotation. So an extra second will be tacked on to Dec. 31 after 6:59:59 p.m. and before 7 p.m. Eastern Standard Time."
"The Earth is rotating slower and slower over time, while the atomic clocks are not slowing down. On one average day the difference is around 0.002 seconds,which means around 1 second in 500 days. In order to synchronize the atomic clocks with the Earth's observed rotation, the atomic clocks are occasionally instructed to add an extra second – the leap second. Leap seconds are inserted so that the difference between the UTC (Coordinated Universal Time) and UT1 (mean solar time - observed Earth rotation) is kept below 0.9 seconds. "
Time and date.com