GMT (Greenwich Mean Time)
The mean solar time at the Royal Observatory in Greenwich, London.
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London. It was historically used as the international civil time standard before being replaced in scientific contexts by Coordinated Universal Time (UTC).
GMT was established in 1675 when the Royal Observatory was built as a reference time for nautical purposes and the British navy. It became the global time standard in 1884 at the International Meridian Conference, where the Greenwich meridian was adopted as the Prime Meridian (0° longitude).
Today, GMT is still used in some contexts, particularly in the United Kingdom during winter months, but has largely been superseded by UTC for scientific and technical purposes. The term GMT is often used informally to refer to UTC+0, though technically they can differ by up to 0.9 seconds due to leap seconds and other adjustments in UTC.
Unlike UTC, which is based on atomic time, GMT is based on astronomical observations and the rotation of the Earth, which is slightly irregular.