The time at which summer time begins and ends is given in the relevant EU Directive and UK Statutory Instrument as 1 a.m. Greenwich Mean Time (GMT). In practice*, in the UK, summer time starts when 00:59:59 UTC is followed by 02:00:00 BST (British Summer Time) and it ends when 01:59:59 BST (when it occurs for the first of two times separated by one hour) is followed by 01:00:00 UTC. In all other EU countries, and many other European countries, the clocks are going forward and back at the same instant as in the UK. For example, the hour moves between 1 and 2 also in Ireland and Portugal, while at the same time it is moving between 2 and 3 in France and Germany and between 3 and 4 in Finland and Greece. The dates for summer time have been common throughout all EU countries since 22 October 1995.

Before 1981 the changes to and from summer time in the UK were made one hour later.

*All time signals are based on Coordinated Universal Time (UTC), which can be almost one second ahead of, or behind, GMT so there is a brief period in the UK when the directive is not being strictly followed.