The United Kingdom uses GMT (UTC+0) in winter and BST (UTC+1) in summer, switching on the last Sunday in March and reverting on the last Sunday in October. IANA identifier: Europe/London.

Key facts about time in the United Kingdom

  • Timezone: Greenwich Mean Time (GMT) / British Summer Time (BST)
  • UTC offset: UTC+0 (winter), UTC+1 (summer)
  • DST: Yes, clocks advance one hour at 01:00 GMT on the last Sunday in March, and revert at 01:00 GMT on the last Sunday in October
  • IANA identifier: Europe/London
  • Capital: London

The Summer Time Act 1972, Section 1(2), defines the period of summer time precisely: it begins “at one o’clock, Greenwich mean time, in the morning of the last Sunday in March” and ends “at one o’clock, Greenwich mean time, in the morning of the last Sunday in October.” That provision was substituted by Statutory Instrument 2002/262 on 11 March 2002, aligning the UK’s schedule with the EU standard. The UTC offset during summer time is GMT+1.

GMT is the mean solar time at the longitude of the Royal Observatory Greenwich. It is closely related to UTC but not identical. UTC is maintained by atomic clocks coordinated internationally, and international agreements require UTC to remain within 0.9 seconds of UT1 (a measure of Earth’s rotation). The UK’s National Physical Laboratory operates the national time scale UTC(NPL) and contributes to international UTC. For civil and everyday purposes, GMT and UTC are treated as interchangeable.

The observatory on the hill

The Royal Observatory Greenwich was established in the 1670s, its purpose to improve the precision of astronomical observation for maritime navigation, specifically to help determine longitude at sea. John Flamsteed served as the first Astronomer Royal, and his successors continued systematic astronomical recording for two centuries. By the mid-19th century, Greenwich had accumulated records of precision sufficient to make it the reference point for British maritime charts, and Greenwich Mean Time was used by navigators operating under the British flag.

The International Meridian Conference

In 1884, delegates from multiple nations met at the International Meridian Conference in Washington DC. They voted to designate Greenwich as the prime meridian, longitude 0 degrees, the reference point for all longitude measurements. France did not vote in favour and continued using the Paris meridian on official French maps for several decades afterward. The practical rationale for Greenwich was that a significant portion of global shipping already used charts based on the Greenwich meridian, so standardising on Greenwich disrupted the fewest vessels.

British Standard Time: the 1968-1971 experiment

After World War II the UK settled into a GMT/BST cycle. In 1968, the British Standard Time Act established a trial of year-round UTC+1 (clocks fixed one hour ahead of GMT, with no seasonal change). The IANA timezone database records this precisely: the Zone Europe/London definition shows 0:00 GB-Eire %s until 1968 Oct 27, then 1:00 - BST until 1971 Oct 31, confirming the UK remained at UTC+1 continuously for that period. The trial ended on 31 October 1971 and the country reverted to the standard GMT/BST cycle.

Wartime double summer time

During and after World War II, the UK used British Double Summer Time (BDST, UTC+2) in the warmer months while retaining BST in winter, abandoning the return to GMT entirely for several years. The IANA database records BDST transitions in 1941, 1942 to 1944, 1945, and 1947. The abbreviation BDST appeared in official wartime correspondence, including communications cited in the tz database commentary. After 1947, the double offset was not repeated.

EU alignment and post-Brexit position

From 1996, the Zone Europe/London definition switched from GB-Eire rules to EU rules, meaning UK DST transitions have been governed by the same EU schedule since then. The EU standardized the last-Sunday-in-March and last-Sunday-in-October rule from 1981 onward. Post-Brexit, the UK has retained this schedule but is no longer formally bound by EU regulations on the matter. A permanent change to clocks could theoretically be enacted by Parliament.

For developers

The IANA timezone for the UK mainland is Europe/London. The hasDST flag is true. Transitions occur on the last Sunday in March (UTC+0 to UTC+1) and the last Sunday in October (UTC+1 to UTC+0), both at 01:00 local time. Note that the 1968-1971 period, the wartime BDST years, and the pre-1996 GB-Eire rule set all appear in the tz database and may affect historical timestamp calculations.

Sources