Time in the United Kingdom
The islands of Great Britain and Ireland used Local Mean Time until railway timetabling gradually established the two standards of Greenwich Mean Time and Dublin Mean Time (UTC-00:25, no longer used). These were officially adopted under the Statutes (Definition of Time) Act 1880 (43 & 44 Vict.). By 1916, Greenwich Mean Time was introduced to Ireland.
Daylight saving time was introduced by the Summer Time Act 1916 (6 & 7 Geo. V), which was implemented in 1916. The length of DST could be extended by Order in Council, and was extended for the duration of World War I. For 1916, DST extended from 21 May to 1 October, with transitions at 02:00 standard time.
Following the enactment of The Summer Time Act 1972 the last Sunday in March became the start of British Summer Time with the last Sunday in October being end of British Summer Time. 
In addition to GMT, since 1997 and the Hong Kong Handover the UK has operated a total of 7 time zones, covering its possessions in the Mediterranean and Caribbean seas and South Atlantic, Pacific, Indian and Southern Oceans.
Experimentations with Central European Time
The United Kingdom experimentally adopted Central European Time by maintaining Summer Time throughout the year from 1968 to 1971. In a House of Lords debate, Richard Butler, 17th Viscount Mountgarret said that the change was welcomed at the time, but the experiment was eventually halted after a debate in 1971, in which the outcome might have been influenced by a major accident on the morning of the debate.
Regulation of timing is a reserved matter, meaning that only the Parliament of the United Kingdom has power to legislate, rather than devolved administrations such as the Scottish Parliament except the Northern Ireland Assembly which has the powers over time zones but has never used the power.
List of UTC offsets in the UK, its territories and Crown Dependencies
- UTC−08:00 — Pitcairn Islands
- UTC−05:00 — Cayman Islands, Turks and Caicos Islands
- UTC−04:00 (AST) — Anguilla, Bermuda, British Virgin Islands, Montserrat
- UTC−03:00 (FKST) — Falkland Islands
- UTC−02:00 — South Georgia and the South Sandwich Islands
- UTC (GMT) — the United Kingdom, Channel Islands, Isle of Man, Saint Helena, Ascension and Tristan da Cunha
- UTC+01:00 (CET) — Gibraltar
- UTC+02:00 (EET) — Akrotiri and Dhekelia
- UTC+06:00 — British Indian Ocean Territory
IANA time zone database
This refers to the area having the ISO 3166-1 alpha-2 country code "GB".
- Myers J (5 October 2008). "History of legal time in Britain". Retrieved 15 July 2009.
- "Fresh attempt to change UK time". BBC News. 25 January 2007. Retrieved 19 January 2010.
- "Central European Time Bill [H.L.] HL Deb 11 January 1995 vol 560 cc243-84". Hansard. 1995. Retrieved 19 January 2010.
- "Tundra time call in clocks debate". BBC News. 23 October 2009. Retrieved 19 January 2010.
- "Plan to bring UK clocks forward". BBC News. 20 February 2011. Retrieved 20 February 2011.