Time Zone Calculators

Instants, offsets and wall-clock time

Three ideas explain almost every time zone confusion.

• An instant is a single moment, the same everywhere. A Unix timestamp is an instant.
• An offset is how far a place is from UTC at a given moment — London is UTC+0 in January and UTC+1 in July.
• A wall-clock time is what a clock in that place reads. It only identifies an instant once you know the offset that applied.

Because offsets change, a wall-clock time is not a fixed thing. That is why these calculators apply the offset in force on the date you enter rather than today's.

The two hours that break calendars

The hour that never happens

When clocks go forward, an hour is skipped. In the UK on 29 March 2026 the clock jumps from 00:59 to 02:00, so 01:30 does not exist. Software that assumes every wall-clock time is valid will either throw an error or silently produce the wrong instant. The time zone converter tells you, and returns the instant the clocks moved to.

The hour that happens twice

When clocks go back, an hour repeats. In the UK on 25 October 2026 the hour from 01:00 to 02:00 runs twice, once on summer time and once on standard time. A meeting at 01:30 that day is genuinely ambiguous, and the two possibilities are a full hour apart. Both are shown.

Why recurring meetings drift

Countries change their clocks on different dates. The United States moves in early March and early November; the UK and EU move in late March and late October. For roughly three weeks each year the usual gap between London and New York is four hours instead of five. A weekly call that has worked all summer will land an hour out for part of the spring and autumn.

The practical advice: check any recurring meeting against the actual date after each transition, using the meeting time planner.

Where the data comes from

Time zone rules come from the IANA time zone database built into your browser — the same source used by operating systems and most programming languages. Because it comes from your device rather than from this site, it stays current as your browser updates, which matters because governments change daylight-saving rules at short notice.