Time zone calculator
Doing it by hand takes three steps and works every time — provided you handle the two things mental arithmetic reliably drops.
The short answer
Convert to UTC, then out again. Take the source time, subtract its offset to get UTC, then add the destination's offset.
Going via UTC rather than subtracting one offset from the other is what keeps you right when the two sides are on different daylight-saving schedules.
The method
14:00 in Berlin, what is that in Mumbai, in July?
- To UTC. Berlin in July is UTC+2. Subtract two hours: 12:00 UTC.
- Out to the destination. Mumbai is UTC+5:30 year-round. Add five and a half hours: 17:30.
- Check the date. 17:30 is still the same day. Done.
The reason to route through UTC rather than computing the gap directly: each side's offset depends on its own date and its own rules. Handling them separately means you never have to reason about two daylight-saving schedules at once.
The two things mental arithmetic drops
The date. Any result below 00:00 or above 24:00 rolls over.
20:00 New York, to Tokyo, in July. To UTC: +4 → 00:00 UTC, next day. To Tokyo: +9 → 09:00, next day.
If you skip the rollover you get the hour right and the day wrong, which is worse than being an hour out.
The half hours. India is +5:30, Nepal +5:45, Iran +3:30. Rounding to whole hours is a habit worth actively suppressing — the error is small enough to look plausible and large enough to miss a meeting.
Getting the offsets right
The offset is a property of a place and a date together, not of a place.
| Place | Winter | Summer |
|---|---|---|
| New York | UTC−5 | UTC−4 |
| London | UTC+0 | UTC+1 |
| Berlin | UTC+1 | UTC+2 |
| Mumbai | UTC+5:30 | UTC+5:30 |
| Tokyo | UTC+9 | UTC+9 |
| Sydney | UTC+11 | UTC+10 |
Note Sydney: southern hemisphere, so its "winter" column is its summer offset. This is the classic way a hand calculation goes wrong.
When to stop doing it by hand
Any of these, and you should let software resolve it:
- The date is more than a couple of weeks out.
- The date is near a transition — mid-March, late October, early November.
- Either side is somewhere you are not certain about the DST rules for.
- Money, contracts or travel depend on the answer.
Hand calculation is for a quick sanity check. It is not for a deadline.
Related: Time zone converter · Time difference calculator
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.