BST — British Summer Time

British Summer Time (BST) is one hour ahead of Coordinated Universal Time and is the daylight saving time variant used by the United Kingdom during the summer months. BST begins on the last Sunday in March, when clocks are set forward one hour from GMT (UTC+0) to BST (UTC+1), and ends on the last Sunday in October, when clocks return to GMT. Ireland observes the same clock change but calls its summer time Irish Standard Time (IST).

BST was first introduced in the United Kingdom through the Summer Time Act of 1916, during World War I, following a campaign by builder William Willett who argued that advancing clocks in summer would save energy and give people more usable daylight in the evenings. The UK briefly experimented with year-round BST (called British Standard Time) from 1968 to 1971, but the experiment was ended after concerns about darker winter mornings, particularly in Scotland and northern England.

During BST, the United Kingdom is on the same time as Central European Time (CET), aligning London's business hours more closely with those of Paris, Berlin, and other continental European capitals. This seasonal alignment is valued by businesses operating across the English Channel. The question of whether to adopt permanent summer time or permanent winter time has been debated periodically, particularly in the context of the EU's 2019 proposal to end seasonal clock changes.