GMT — Greenwich Mean Time

Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, and has historically served as the global reference point for timekeeping. It is equivalent to UTC+0 and is used by the United Kingdom, Ireland, and Portugal during the winter months, as well as by several West African countries year-round. The prime meridian, zero degrees longitude, passes through Greenwich and defines the boundary from which all other time zones are measured.

GMT was established in 1675 when the Royal Observatory was founded to aid maritime navigation. By the late 19th century, as global communication and rail travel demanded standardized timekeeping, GMT was adopted as the world's time standard at the International Meridian Conference of 1884. Although Coordinated Universal Time (UTC) replaced GMT as the official world time standard in 1972, the two are often used interchangeably in casual usage, and GMT remains the legal time standard in the United Kingdom.

During the summer months, the United Kingdom and Ireland switch to British Summer Time (BST, UTC+1) and Irish Standard Time (IST, UTC+1), respectively. Portugal and Iceland, however, remain on GMT year-round. The Canary Islands, a Spanish territory in the Atlantic, also use GMT rather than the CET observed by mainland Spain.