Current Time in London

The rule behind those dates is simple. British Summer Time begins at 1:00 AM GMT on the last Sunday in March, when clocks jump to 2:00 AM, and ends at 2:00 AM BST on the last Sunday in October, when they drop back to 1:00 AM. The spring night is one hour shorter and the autumn night one hour longer.

Phones and computers apply the change by themselves, using the shared time zone database that also drives this page. A clock that does not update, such as an oven or a car dashboard, has to be set by hand twice a year.

The two columns differ wherever one city uses daylight saving time and the other does not, or where the seasons are reversed. Sydney is a good example: it is ahead of London by eleven hours in January, during the Australian summer, but by nine hours in July.

The US and the UK both use summer time but start it on different Sundays. The US changes on the second Sunday in March and the first Sunday in November, the UK on the last Sundays of March and October, so New York sits four hours behind London for a short time in each season instead of five. For calls with India or Chicago, the IST to CST converter handles the same kind of shift.

GMT takes its name from the Royal Observatory in Greenwich, in south-east London, where the prime meridian of longitude was set in 1884. For most of the twentieth century it was the world's reference time.

Today that role belongs to UTC, Coordinated Universal Time, which is kept by atomic clocks. In everyday use GMT and UTC show the same time, but UTC is the term used in aviation, computing and science, and it never shifts for summer.