GMT

Greenwich Mean Time (GMT) is a time standard rooted in the mean solar time at the prime meridian, which passes through Greenwich, London. It originated as a navigation and astronomy reference and served as the basis for global timekeeping for many years, with the 0 degree longitude used to define time in relation to the sun’s position.

Historically, GMT gained prominence after the Royal Observatory in Greenwich was used to establish a single reference meridian. The 1884 International Meridian Conference formally designated the Greenwich meridian as the prime meridian and promoted GMT as the global standard for timekeeping. Over time, GMT became embedded in maritime, scientific, and civil timekeeping practices.

GMT and UTC relate but are not identical. Coordinated Universal Time (UTC) is the modern primary time standard, based on highly precise atomic clocks and adjusted with leap seconds to stay aligned with Earth’s rotation. GMT is often used to denote UTC+0 in everyday language or as a time zone, but it is conceptually mean solar time, which can drift relative to atomic time. In practice, many systems treat GMT as equivalent to UTC+0 for routine purposes, though the two have different technical foundations.

Usage today varies by region. GMT remains a civil time zone in some areas (for example, parts of Africa and some territories) and is used in the United Kingdom when not observing daylight saving time; during summer the UK uses British Summer Time (BST), which is GMT plus one hour. Other regions observe GMT year-round.

See also: UTC, Greenwich Observatory, Prime Meridian, leap seconds.