History
British and American daylight saving time: a short history
Britain and the United States both move clocks seasonally, but their schedules and legal histories are different—and that difference matters for transatlantic meetings.
British Summer Time is a specific civil rule
In the UK, clocks move forward by one hour at 01:00 on the last Sunday in March and back at 02:00 British Summer Time on the last Sunday in October. In winter, the UK uses Greenwich Mean Time.
The United States has its own calendar
The US daylight-saving schedule is set separately. Most participating locations change clocks on different March and November dates from the UK, so London and New York do not keep the same time difference throughout the transition weeks.
This is why dates matter
“London is five hours ahead of New York” is usually useful, not permanently true. Schedule a meeting with both cities and the actual date; a converter using current time-zone rules can then account for the awkward weeks when one country has changed clocks and the other has not.