Pick two cities and a moment. The clocks below show the same instant in both places, with daylight saving already applied.
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A time zone is a legal decision, not a line drawn by the sun. China spans five geographic hours and runs on one clock; France has twelve zones because of its overseas territories; Nepal sits at UTC+5:45 to place solar noon over Kathmandu rather than over India. None of that follows from longitude alone, which is why a converter reads a database of laws rather than dividing degrees by fifteen.
The rules also change. Brazil ended daylight saving in 2019, Mexico ended most of it in 2022, and the European Union has been debating abolition since 2018. Your browser ships the current database, and this page reads it — that is why the answer can differ from a printed table made a few years ago.
The practical consequence: never store a meeting as a fixed offset. "São Paulo is three hours behind London" is true today and false in December. Store the instant, or the local time plus the zone, and let the rules resolve it.
Because daylight saving does not start and end on the same date everywhere — and some places do not observe it at all. São Paulo dropped it in 2019, so the gap to London is three hours for part of the year and four for the rest. That is why this page asks for a date instead of showing one fixed number.
On the night the clock jumps forward, an hour is skipped — 02:30 simply never happens. The page says so and shows the next valid time instead of pretending the input was fine.
When the clock falls back, the same hour repeats. We show the first occurrence, which is the standard convention, and say that the time is ambiguous.
They use the IANA time zone database that ships with your browser, which encodes the historical rules. Future dates assume current law — a government can change the rules with a few months' notice, and no calculator can predict that.
Find your exact age in years, months and days — and how long until your next birthday.
Count the days, weeks and months between any two dates.
How many days are left until a holiday, a deadline or any future date.
Count business days between dates, or find when a deadline in working days lands.
Which week of the year a date falls in, and the week it belongs to.
The sequential day number, how many remain, and how much of the year has passed.