Converting through UTC
Every conversion goes through Coordinated Universal Time (UTC), the reference time scale kept by atomic clocks. Each place on Earth keeps local time at a fixed distance from UTC, called its UTC offset: New York is UTC−5 in winter, Tokyo is UTC+9 all year. To convert a time, the calculator removes the “From” zone’s offset to get UTC, then applies the “To” zone’s offset.
The offset is not a property of a city alone. It depends on the date, because many places move their clocks for daylight saving time and governments change their rules from time to time. That is why the converter asks for a date as well as a time, and why it shows both offsets that were in force at that moment.
The rules come from the IANA Time Zone Database, which records the history of local time for representative places around the world. Your browser carries a copy of it, so nothing is fetched when you convert; keeping the browser updated keeps the rules current.
The formula
The arithmetic is exact. The only uncertainty is in the rules themselves: a converted time far in the future assumes that today’s daylight saving rules will still apply then.
Zone names and offsets that are not whole hours
Zones are listed by IANA names in the form Area/Location, such as America/New_York, Europe/London or Asia/Kolkata. Abbreviations are avoided because they are ambiguous and they flip with the seasons: IST can mean India, Ireland or Israel, and New York is on EST only part of the year.
Most offsets are whole hours, but several are not:
| Place | IANA zone | Offset in January | Offset in July |
|---|---|---|---|
| India | Asia/Kolkata | UTC+5:30 | UTC+5:30 |
| Nepal | Asia/Kathmandu | UTC+5:45 | UTC+5:45 |
| Iran | Asia/Tehran | UTC+3:30 | UTC+3:30 |
| Newfoundland, Canada | America/St_Johns | UTC−3:30 | UTC−2:30 |
| South Australia | Australia/Adelaide | UTC+10:30 | UTC+9:30 |
| Chatham Islands, New Zealand | Pacific/Chatham | UTC+13:45 | UTC+12:45 |
The South Australia and Chatham rows have their larger offset in January because those places keep daylight saving time through the southern summer, roughly October to April, the opposite season to Europe and North America.
Daylight saving: different dates, skipped and repeated hours
In the parts of the United States that use daylight saving time, it starts at 2:00 local time on the second Sunday in March and ends at 2:00 on the first Sunday in November. The United Kingdom moves its clocks forward at 1:00 on the last Sunday in March and back at 2:00 on the last Sunday in October, and EU countries switch on those same Sundays. Between the American and the European dates, two or three weeks in spring and one week in autumn, the gap between New York and London is 4 hours instead of the usual 5.
When clocks jump forward, one hour of local time never happens. In New York, 2:30 on March 8, 2026 does not exist. The converter warns you and moves the time forward by the size of the jump, to 3:30. When clocks go back, an hour happens twice: 1:30 on November 1, 2026 occurs once at UTC−4 and again an hour later at UTC−5. The converter warns you and uses the first occurrence.
Worked examples
- New York is on daylight saving time, UTC−4, so 9:00 there is 13:00 UTC.
- Add India’s offset: 13:00 + 5:30 = 18:30.
- Add Nepal’s offset: 13:00 + 5:45 = 18:45.
- Los Angeles is at UTC−7 in October, so 18:00 there is 01:00 UTC on October 14.
- Tokyo is UTC+9 with no daylight saving: 01:00 + 9 = 10:00 on October 14.
- October 13: New York UTC−4, London UTC+1, a 5-hour gap, so 14:00.
- October 27: London went back to UTC on October 25, New York is still UTC−4, a 4-hour gap, so 13:00.
- November 3: New York went back to UTC−5 on November 1, and the 5-hour gap returns, so 14:00.
Mistakes to avoid when converting time zones
- Using a fixed difference all year. The New York to London gap is 5 hours most of the year but 4 hours for a short spell around each clock change, as the third example shows.
- Ignoring the date line. An evening in the Americas is already the next morning in East Asia and Australia. Read the date in the result, not just the time.
- Typing a fixed offset for a place that changes clocks. UTC−5 is New York’s offset only in winter. Choose the zone by name and the season is handled for you.
- Scheduling inside a skipped hour. An alarm or meeting set for 2:30 on the night clocks go forward has no matching moment. Pick a time outside the change window.
Time zone rules change when governments decide to change them, and the database is updated to match. If a country has announced a change very recently, an older browser may not know it yet.
Questions people ask
Is GMT the same as UTC?
For everyday clock times they match: GMT is the name of the UK’s winter time, at offset zero. UTC is the precise international time scale, based on atomic clocks.
Why does the converter preselect my zone?
Your browser reports the IANA zone your device is set to, and the converter puts it in the “From time zone” field. The “To” field starts on London, or on New York if your own zone is London. Change either one, or use Swap zones.
Sources
- Time Zone Database, Internet Assigned Numbers Authority (IANA).
- Daylight Saving Time Rules, National Institute of Standards and Technology.
- When do the clocks change?, GOV.UK.
- Summertime, European Commission, Mobility and Transport.