Summer Time Act 1916

The United Kingdom uses Greenwich Mean Time or Western European Time (UTC) and British Summer Time or Western European Summer Time (UTC+01:00).

Until the advent of the railways, the United Kingdom used Local Mean Time. Greenwich Mean Time was adopted first by the Great Western Railway in 1840 and a few others followed suit in the following years. In 1847 it was adopted by the Railway Clearing House, and by almost all railway companies by the following year. It was from this initiative that the term "railway time" was derived.

It was gradually adopted for other purposes, but a legal case in 1858 held "local mean time" to be the official time. On 14 May 1880, a letter signed by 'Clerk to Justices' appeared in 'The Times', stating that 'Greenwich time is now kept almost throughout England, but it appears that Greenwich time is not legal time. For example, our polling booths were opened, say, at 8 13 and closed at 4 13 PM.' This was changed later in 1880, when Greenwich Mean Time was legally adopted throughout the island of Great Britain under the Statutes (Definition of Time) Act 1880 (43 & 44 Vict.). GMT was adopted on the Isle of Man in 1883, Jersey in 1898 and Guernsey in 1913. Ireland adopted GMT in 1916, supplanting Dublin Mean Time.

Daylight saving time was introduced by the Summer Time Act 1916 (6 & 7 Geo. V), which was implemented in 1916 as GMT plus one hour and Dublin Mean Time plus one hour. 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. On 1 October 1916, Greenwich Mean Time was introduced to Ireland.

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