Coordinated Universal Time

Coordinated Universal Time (abbreviated UTC) is the basis for standard time in time zones around the world. It is a compromise between a measurement of time based solely on an atomic clock and a one based solely on the rotation of the Earth.

Because the day is defined as exactly 86,400 seconds (where the second is defined in terms of a physical property of atoms of the element cesium), but the Earth takes about 86,400.002 seconds to make exactly one rotation, time kept solely according to an atomic clock would eventually get out of synchronization with time as measured by astronomical observations. But to keep time strictly according to the period of the Earth's rotation -- which varies slightly from day to day -- would require resetting clocks by differing tiny fractions of a second every day. As a compromise, UTC, introduced in 1972, is defined so as to never be more than 0.9 seconds behind or ahead of the time as determined by astronomical observation (called "Universal Time," "UT," or, unofficially, "Greenwich Mean Time").

Because the difference between UT and UTC builds up to about one second about every 500 days, "leap seconds" are added to the end of the last minute of July or December in certain years. There have been 33 leap seconds added since UTC began; the most recent was at the end of December 2005, and the next will be at the end of December 2008. (There can also be negative leap seconds, if UTC were to get ahead of rather than behind UT, but so far this has never happened.)

UTC is also, by definition, always an integral number of seconds (no fractions) different from International Atomic Time (TAI), a measurement based solely on atomic clocks, which is never adjusted to account for variations in the Earth's rotational speed.

UTC is the time broadcast by time-signal stations such as WWV. It is also the time used by the Global Positioning System (GPS).