Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, counted from midnight. It is the local clock time at Greenwich and the international standard of civil time from 1884 until 1972. GMT is also used as one of the names for the time zone UTC+00:00. It is the mean (average) solar time at the Greenwich Meridian or Prime Meridian, which is 0 degrees longitude. GMT is considered equivalent to UT1, the modern form of mean solar time at 0° longitude, for navigation purposes. However, this meaning can differ from UTC by up to 0.9 seconds. The term GMT should not be used for purposes that require precision.

To obtain local time in the United States, a certain number of hours must be subtracted from UTC depending on how many time zones away from Greenwich one is located. The switch to daylight saving time does not affect UTC. The term GMT is still used to represent civil time in Britain.