Coordinated Universal Time (UTC) is the basis for civil time in many places worldwide. Many timekeeping devices use this 24-hour time standard, which is determined using highly precise atomic clocks. The hours, minutes, and seconds that UTC expresses is kept close to the mean solar time at the Earth's prime meridian (zero degrees longitude) located near Greenwich, England.
http://www.timeanddate.com/time/aboututc.html
Universal Time (UT) is a time standard based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the Prime Meridian at Greenwich, and GMT is sometimes used loosely as a synonym for UTC. In fact, the expression "Universal Time" is ambiguous (when accuracy of better than a few seconds is required), as there are several versions of it, the most commonly used being UTC and UT1 (see below).[1] All of these versions of UT are based on the rotation of the Earth in relation to distant celestial objects (stars and quasars), but with a scaling factor and other adjustments to make them closer to solar time.
http://en.wikipedia.org/wiki/Universal_Time
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