The acronym stands for Greenwich Mean Time (GMT). It is now better known as Universal Time (UT). This is local civil time at the former site of the Royal Observatory in Greenwich, England, located on the prime meridian.
This term is used as the basis for standard time in most countries around the world. This was decided at the 1884 International Meridian Convention in Washington (USA).
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Civil time can be formally defined as mean solar time plus 12 hours. The civil day begins at midnight, while the mean solar day begins at noon. Civil time is occasionally adjusted in one-second increments.
This is to ensure that the difference between a uniform time scale defined by atomic clocks does not differ from the Earth's rotation time by more than 0.9 seconds. Coordinated Universal Time (UTC), atomic time, is the basis of civil time.
Civil time is generally not used as it depends on the observer's longitude; instead, standard time, which is the same throughout a given time zone, is generally adopted.
Standard time is the civil time used within a given time zone. The Earth is divided into 24 time zones, each about 15° longitude, which corresponds to one hour. Within a zone, all civil clocks are set to the same local solar time.
Adjacent zones usually differ by a full hour, although there are exceptions. Standard time is based on universal time. Standard time was largely the brainchild of Canadian railway engineer Sir Sandford Fleming (1827–1915).
Its establishment was due primarily to the efforts of educator Charles Dowd and William Allen, secretary of the American Railroad Association. The official time officially came after a White House meeting in 1984, in 1884 with the prime meridian established in Greenwich, England.
