Time Zone Converter

Time zones, and the weeks when the usual difference is wrong

Converting between time zones is simple arithmetic on UTC offsets. What catches people out is that those offsets change on different dates in different countries, so for a few weeks each year the familiar difference is off by an hour.

How it works

• Converts a time to UTC using the source zone's current offset, then applies the target zone's offset.
• Uses the offset in force on the specific date, not a fixed number, so daylight saving is handled correctly.
• Works from named zones rather than raw offsets, which is what makes historical and future dates reliable.

UTC = local time − source offset
local = UTC + target offset

offsets change with daylight saving:
  EU switches on the last Sunday of March and October
  US switches on the second Sunday of March and first Sunday of November
  Japan, India and most of Asia do not switch at all

Worked example

London to New York on two dates in 2026 — 20 March and 20 July — for a call at 15:00 UTC.

1. 20 March: London 15:00 GMT, New York 11:00 EDT
2. the difference is 4 hours, because the US has switched and the EU has not
3. 20 July: London 16:00 BST, New York 11:00 EDT
4. now the difference is the familiar 5 hours

For roughly three weeks in March and one in autumn, London and New York are four hours apart rather than five. A recurring meeting set by fixed local times silently moves for everyone on one side of the Atlantic.

Reading the result

• Store and transmit instants in UTC, and convert only for display. A meeting stored as a local wall-clock time will drift when either participant's zone changes; one stored as an instant will not.
• Use zone names, not offsets. 'Europe/London' is correct across daylight saving and across historical rule changes; 'UTC+1' is only correct for part of the year and tells you nothing about which part.
• Not every offset is a whole hour. India is UTC+5:30, Nepal UTC+5:45, and parts of Australia UTC+8:45 or UTC+9:30. Arithmetic that assumes whole hours will be wrong for around two billion people.
• The southern hemisphere runs daylight saving in the opposite half of the year, so the gap between London and Sydney varies between nine, ten and eleven hours depending on the date.

Common questions

Why did my recurring meeting shift by an hour?

Almost certainly a daylight saving transition that one participant's zone made and the other's did not. The EU and US switch on different dates in both spring and autumn, leaving roughly four weeks a year when the usual difference is an hour smaller.

What is the safest way to schedule across zones?

Agree the instant in UTC, and let each calendar render it locally. If you must state it in words, name the zone and the date rather than the offset — '15:00 UTC on 20 March' is unambiguous where '4pm CET' is not.

Sources

• IANA Time Zone Database