Time zone converter: what a good one actually does
Converting an hour is trivial. The difference between converters is entirely in the four cases that are not trivial.
The short answer
A converter takes one moment in one place and expresses it in another. The arithmetic is subtraction; the difficulty is entirely in the edge cases.
Four things separate a reliable converter from a wrong one: half-hour zones, daylight saving on both sides, date rollover, and transitions between now and the date you selected.
The four hard cases
Half- and quarter-hour offsets. India is UTC+5:30, Nepal UTC+5:45, Chatham UTC+13:45. Anything doing whole-hour arithmetic is wrong by up to 45 minutes for a very large number of people.
Daylight saving on both sides. The gap between two places depends on the DST state of each. If one observes it and the other does not, the gap moves twice a year — New York to Mumbai is 10:30 hours ahead in winter and 9:30 hours ahead in summer.
Date rollover. For anything more than about eight hours away, the answer frequently lands on a different calendar day. A converter that returns a time without a date has withheld the half that causes real mistakes.
The date you picked, not today. This is the one most often wrong. Converting a date three weeks out must use the rules in force then, which may differ from now — especially across a transition. A converter that applies today's offset to a future date is wrong for exactly the appointments people care most about.
What the output should include
- The converted time and date, with today/tomorrow/yesterday made explicit.
- The offset of each side on that date, not in general.
- A flag when the selected date sits near a transition, because that is when hand-checking is worth it.
- Ideally, whether it is day or night there — the real question is usually "is this a reasonable hour for them," not the number.
What it cannot tell you
A converter answers "what time is X there." It does not answer "when should we meet" — that is a different computation over two ranges rather than one point, and it is the question most scheduling actually needs.
For that you want an overlap calculation, which finds the window where both parties are awake rather than converting a moment you have already guessed.
Testing one
Convert 10am IST to EST for a date in January, then a date in July.
A correct converter gives two different answers (10:30 hours ahead and 9:30 hours ahead apart), handles the 30-minute offset exactly, and tells you the date rolls back. One that gives a single rounded answer has failed three of the four hard cases at once.
Related: Time zone calculator · When are two cities both awake?
Timezone: World Clock Widget
A world clock, converter and live day/night map for iPhone — 325 cities, offline, no account.
Figures on this page are computed from the IANA time zone database. Last generated 2026-08-18.