Greenwich Mean Time (London)

Greenwich Mean Time (GMT) is the historic baseline timezone from which all other world timezones are measured. The Royal Observatory in Greenwich, London, established the prime meridian (0° longitude) in 1884 at the International Meridian Conference. During winter, the UK observes GMT (UTC+0), and during summer it switches to British Summer Time (BST, UTC+1), changing on the last Sunday of March and October. London is one of the world's leading financial centers — the London Stock Exchange operates on GMT/BST and the city's position between Asian and American markets makes it the global hub for foreign exchange trading. The UK's timezone has been remarkably stable, though there was a brief experiment with British Standard Time (UTC+1 year-round) from 1968-1971, abandoned due to darker Scottish mornings. GMT remains the common reference point in aviation, shipping, and international law. Many programming languages and operating systems use GMT or UTC as their default timezone. The UK's timezone is shared with Ireland (IST), Portugal (WET), and Iceland (which uses GMT year-round without DST).