A postcard postmarked “21:30, 15 July 1942” lands in a family archive. July sun in London sets after 21:00, and the modern rule is British Summer Time at UTC+1. That postmark is not a mistake. It is British Double Summer Time: a clock two hours ahead of Greenwich Mean Time, official, real, and in force for seven years.
What British Double Summer Time Was
British Double Summer Time (BDST) was the daylight saving scheme the United Kingdom used during the Second World War and for a short period after it. Under BDST, clocks advanced two hours ahead of Greenwich Mean Time. The offset was UTC+2.
The standard rule today (BST at UTC+1 from the last Sunday in March to the last Sunday in October) was set aside. From 1940 until 1947, the country lived in a permanent summer-time-plus-one arrangement: clocks on UTC+2 during the warmer months, UTC+1 (what we now call BST) during winter. No autumn clock change back to GMT until the experiment ended.
Why the Country Did It
Fuel and production. The same reason British Summer Time entered law in 1916. Germany adopted daylight saving in April 1916. The UK followed a month later with the Summer Time Act 1916. Shift an hour of daylight from the early morning, when most people were asleep, to the evening, when factories and farms could use it. More daylight meant less coal burned for lighting and longer working hours for the war effort.
When the Second World War began, the government went further. In 1940, the Summer Time Order put clocks on BST (UTC+1) year-round. Each summer, they added another hour, creating BDST at UTC+2. Winter remained on UTC+1. A second hour of evening daylight let factories run later without electric light, cut the need for blackout lighting, and gave agricultural workers more time in the fields.
What That Meant for Daily Life
Put yourself in London in June 1941. The summer solstice sunrise is at 04:42 and sunset at 21:21. A working day of 08:00 to 18:00 by the clock started with the sun well up and ended in full light.
The effect was a long, bright evening. People tended allotments, queued for shops, moved through blackout streets in twilight rather than full dark. For children, bedtime came in what felt like midday brightness.
The downside was the morning. In December, under winter UTC+1, the sun rose at 08:03 by today’s clock. Children travelled to school in darkness. Factory shifts started before dawn. Scotland, at a higher latitude, saw even later sunrises, a major reason the later experiment with year-round BST failed in the early 1970s.
When It Ended
BDST was not abandoned as soon as the war ended. The fuel crisis and economic recovery meant the government kept the scheme running through 1945 and 1946.
The final year was 1947. On 2 March 1947, the UK moved to BST (UTC+1) for the winter. On 13 April, clocks advanced to BDST (UTC+2) for the summer. They moved back to BST on 10 August (an early return) and finally back to GMT on 2 November 1947. That was the last time the UK observed a double summer time.
From 1948 onward, the standard pattern resumed: BST in summer, GMT in winter. The Summer Time Act 1972 formalised the modern schedule, aligning the UK with what was then the European Economic Community’s directive.
Could It Happen Again?
A Private Member’s Bill to move the UK to year-round BST (UTC+1) surfaces occasionally in Parliament. The European Parliament voted in 2019 to abolish seasonal clock changes entirely, but the UK confirmed it will not participate. No active government plans exist to change the current system.
If the UK ever adopted year-round BST, winter mornings in northern England and Scotland would be as dark as they were during the BDST winters of the 1940s. The political resistance is strong. British Double Summer Time itself (a UTC+2 offset in summer) would require a further departure from the existing schedule and has no serious support.
The Practical Takeaway for Today
If you are scheduling a call or a meeting with someone in London and you see a reference to “BDST” or “double summer time” in historical documents, remember: that time is two hours ahead of GMT. A meeting at 14:00 BDST would be 12:00 GMT and 13:00 BST.
For current use, the rule is simple. London is on GMT (UTC+0) from the last Sunday in October to the last Sunday in March. It is on BST (UTC+1) for the rest of the year. The next change is on Sunday 28 March 2027 at 01:00, when clocks go forward to 02:00 BST. Until then, London is on GMT.