British Summer Time (BST) is the civil time during the summer months in the United Kingdom during which the clocks are advanced from UTC, also commonly called Greenwich Mean Time (GMT), by one hour (UTC+01:00). The British Summer Time period begins on the last Sunday of March and ends on the last Sunday of October. Though initially popular after its introduction in 1916, the practice now divides opinion.

British Summer Time was first established by the Summer Time Act of 1916, after a campaign by builder William Willett. His original proposal was to move the clocks forward by 80 minutes, in 20-minute weekly steps on Sunday in April and by the reverse procedure in September. At this time BST began on 21 May and ended on 1 October.

In 1940, during the Second World War, the clocks in Britain were not put back by an hour at the end of summertime, and clocks continued to be advanced by one hour each summer until July 1945. During these summers therefore, Britain was 2 hours ahead of GMT and operating on British Double Summer Time (BDST). The clocks were brought back in line with GMT at the end of summer in 1945. In 1947, due to severe fuel shortages, clocks were advanced by one hour on two occasions during the spring, and put back by one hour on two occasions during the autumn, meaning that Britain was back on BDST during that summer.^{[not in citation given]}

Western European Summer Time (WEST) is a summer daylight saving time scheme, 1 hour ahead of Coordinated Universal Time. It is used in the following places:

Western European Summer Time is also known locally, in the countries concerned, as:

The scheme runs from the last Sunday in March to the last Sunday in October each year. At both the start and end of the schemes, clock changes take place at 01:00 UTC. During the winter, Greenwich Mean Time (UTC+0) is used.

The start and end dates of the scheme are somewhat asymmetrical in terms of daylight hours: the vernal time of year with a similar amount of daylight to late October is mid-February, well before the start of summer time. The asymmetry reflects temperature more than the length of daylight.

Ireland does not observe a summer time, but rather observes Standard Time during the summer months and changes to UTC+0 during the winter time period. However, as Ireland's winter time period begins on the last Sunday in October and finishes on the last Sunday in March, the result is the same.

Daylight saving time (DST)—also summer time in several countries, in British English, and European official terminology (see Terminology)—is the practice of advancing clocks so that evenings have more daylight and mornings have less. Typically clocks are adjusted forward one hour near the start of spring and are adjusted backward in autumn.

Though mentioned by Benjamin Franklin in 1784, the modern idea of daylight saving was first proposed in 1895 by George Vernon Hudson and it was first implemented during the First World War. Many countries have used it at various times since then.

The practice has been both praised and criticized. Adding daylight to evenings benefits retailing, sports, and other activities that exploit sunlight after working hours, but can cause problems for evening entertainment and other occupations tied to the sun. Although an early goal of DST was to reduce evening usage of incandescent lighting, formerly a primary use of electricity, modern heating and cooling usage patterns differ greatly, and research about how DST currently affects energy use is limited or contradictory.