Technical Specifications & Standards
Official chronometric properties, ISO standards, and international database registry identifiers for Greenwich Mean Time:
Daylight Saving Time (DST) Transitions
⬆️ Spring Forward (Clocks Advance)
Last Sunday in March at 01:00 AM GMT (Clocks advance to BST, UTC+1)
⬇️ Fall Back (Clocks Return)
Last Sunday in October at 02:00 AM BST (Clocks return to GMT, UTC+0)
Observing Jurisdictions & Subdivisions
24 Sovereign NationsSovereign nations, administrative subdivisions, and regional territories officially operating under GMT:
Regional Subdivisions & Territory Details
Financial Markets & Trading Hours
London overlaps with morning trading in East Asia and afternoon trading on Wall Street, making it the most active session for foreign exchange settlement.
Local Hours: 08:00 - 16:30 GMT
UTC Window: 08:00 - 17:00 UTC
Local Hours: 08:00 - 16:30 GMT
UTC Window: 08:00 - 17:00 UTC
Local Hours: 08:00 - 16:30 GMT
UTC Window: 08:00 - 17:00 UTC
Major Cities in GMT
Primary metropolitan centers and financial hubs observing this time zone:
Adoption History & Astronomical Facts
Year Established: 1847
First used by the Great Western Railway in 1840 and adopted across the British railway network by 1847. Parliament made it the legal standard across Great Britain under the Statutes (Definition of Time) Act 1880. Delegates at the 1884 International Meridian Conference in Washington voted to make Greenwich the Prime Meridian for world navigation.
Geographically, Iceland sits far enough west that it belongs in the UTC-1 or UTC-2 band. Under Act No. 6/1968, the Icelandic parliament moved the country permanently to GMT (UTC+0) to keep banking and flights aligned with London and Scandinavia, meaning solar noon in Reykjavik occurs around 1:30 PM.
Frequently Asked Questions about GMT
What does GMT stand for?
GMT stands for Greenwich Mean Time, based on the mean solar time at the Royal Observatory in Greenwich, London.
Is GMT the same as UTC?
In practice, both show the exact same time (+00:00). Strictly speaking, GMT is a time zone based on Earth's rotation, while UTC is an atomic time standard.
Does the UK use GMT all year?
No. The UK uses GMT from late October to late March, and switches to British Summer Time (BST, UTC+1) for the warmer months.