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What is Greenwich Mean Time (GMT)?

GMT stands for Greenwich Mean Time. It is the time zone at the Royal Observatory in Greenwich, London, which historically served as the world's primary time standard.

Quick Answer

GMT stands for Greenwich Mean Time. It's the time zone based on the Prime Meridian in Greenwich, London. While it was once the world's primary time standard, it has largely been replaced by the more precise Coordinated Universal Time, or UTC. For everyday purposes, GMT and UTC are often considered the same.

The Definition of GMT

GMT stands for Greenwich Mean Time. It is the time zone based on the average time it takes for the Earth to rotate once on its axis, as measured from the Prime Meridian (0° longitude) at the Royal Observatory in Greenwich, London.

Key Facts About GMT:

  • Historical Standard: From 1884 to 1972, GMT was the international civil time standard.
  • Solar vs. Atomic Time: GMT is a solar time standard, based on the sun's position. This differs from UTC, which is based on ultra-precise atomic clocks.
  • Modern Usage: Today, GMT is officially a time zone (UTC+0) used by the UK in winter and several African countries year-round.
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Related Questions

What is UTC (Coordinated Universal Time)?

UTC is the primary time standard by which the world regulates clocks. It's the successor to GMT and is based on atomic time for precision.

What is Daylight Saving Time (DST)?

DST is the practice of advancing clocks by one hour during warmer months to extend daylight into the evening.

How many time zones are in the USA?

The USA and its territories use 9 standard time zones. The 4 contiguous US states time zones are Pacific, Mountain, Central, and Eastern.

What are leap seconds?

A leap second is a one-second adjustment added to UTC to keep it in sync with the Earth's irregular rotation.