London

London keeps Greenwich Mean Time (UTC+0) in winter and British Summer Time (UTC+1) from the last Sunday of March to the last Sunday of October. The clocks change at 01:00 GMT — the same instant as across the European Union — so London stays exactly one hour behind Paris, Berlin and Madrid all year.

Before the railways, every British town kept its own local solar time; Bristol, for example, ran about 10 minutes behind London. Railway companies adopted London time in the 1840s, Greenwich Mean Time became the legal time for Great Britain in 1880, and in 1884 the International Meridian Conference chose the line through the Royal Observatory at Greenwich as 0° longitude. Visitors can still stand on the meridian line in Greenwich Park.

Britain has experimented more than most. During the Second World War it used British Double Summer Time (GMT+2) in summer, and from 1968 to 1971 it kept clocks one hour ahead all year as "British Standard Time" — dropped after complaints about dark winter mornings, especially in Scotland.

Daylight swings widely across the year: in late June the sun sets after 9 p.m., while in December it is dark before 4 p.m. For business, London's morning overlaps with the Asian afternoon and its afternoon with the New York morning — one reason London is the world's largest foreign-exchange trading centre. Remember that the US changes clocks two to three weeks before the UK in March, and a week later in autumn, so for a few weeks New York is only 4 hours behind.