Our current system of civil time is based on coordinated universal time (UTC), which is based on a system of atomic clocks. Leap seconds are used to guarantee that UTC does not differ from the Earth's rotational time by more than 0.9 seconds.

The Earth's rotation is decelerating at a rate of about 1.5 to 2 milliseconds per day per century due to the frictional action of the tides. Currently, the Earth is slower than UTC by about 2 milliseconds per day, so every 450 to 500 days the time difference reaches 0.9 seconds. Leap seconds are added to UTC to keep the two times in agreement. The U.S. Naval Observatory is considered to be the nation's official timekeeper. The U.S. Naval Observatory Master Clock is based on averaged values of 60 independent cesium atomic clocks, as well as some hydrogen maser atomic clocks. On the Internet, visit http://tycho.usno.navy.mil/ to see the Master Clock.