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Universal time, UT

Universal time, or UT, has replaced Greenwich mean time as the official international time standard.  However, except in technical writing, Greenwich mean time may still be used as a synonym for Universal time.

A variant of UT is UTC, or coordinated universal time.   GMT is based on solar time at Greenwich, England; UT, based on sidereal time at Greenwich, is used for astronomical and navigational purposes; and UTC is determined by atomic clocks and is used for civil time in the U.S.

Times in UT are usually given in terms of a 24-hour clock. Greenwich mean time is also called Greenwich time.