We are all familiar with the concept of a leap year. Every four years, an extra day is added to the year to keep the calendar year in sync with the seasonal year. If there were no leap years, then our seasons would be out of sync with our months within a generation! Similarly, a leap second is sometimes added to a year, so that our day is in sync with the Earth's rotation. When such a second is added, the last minute of the last day of the year is a second longer (61 seconds):D. Since 1970s, a total of 32 leap seconds have been added (the last being in 1998), and this year, the 33rd one will be added:).

Check your clocks on New Year's Eve! (Courtesy: University of Oklahoma)Millions of years ago, the Earth had 10 hour long days! Due to tidal and wind forces, gravitational attraction of the moon and planets, and friction within the Earth's mantle and core, the Earth's rotation is slowing down. The extra second will be added to account for this slowdown. The International Earth Rotation and Reference Systems Service in Paris will sneak the extra time in on 31 December 2005.
The advent of atomic clocks in the 1950s allowed for extremely accurate measurement of periods of time. Starting at a particular point in 1958, an international array of these clocks has been counting out seconds, the length of which was defined at that point. This representation of time is the standard by which the public sets their watches. But people have been keeping an eye on changes in the length of seconds, as fractions of the Earth's daily rotation, using astronomical measurements.
Desktop computers will adjust to the added second by talking to other units on the Internet. Our clocks and watches perhaps need to be adjusted manually:):), so do not trust the advertizers when they say that your really accurate atomic clocks do not require any adjustments ;);).