|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
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.