How the Time Zone Calculator Works
The Earth turns, so it is morning in one place while it is night in another. To keep clocks in step with the sun, the world is divided into time zones. If you plan a call, a flight or a live event, you need to know what time it is somewhere else. The Time Zone Calculator converts a time from one place to another, and it takes daylight saving time into account for the date you choose.
What you enter
- The date: it matters, because clocks change during the year in many countries.
- The time: the time on the clock in the place where you start.
- The time zone you start from, and the one you want to convert to. There are more than 40 to choose from, and a swap button reverses the two.
- The clock style: 24-hour or 12-hour.
How the conversion works
Every time zone is defined by how far it is ahead of, or behind, Coordinated Universal Time, which is called UTC. This is the offset. New York in winter is UTC-5, and Tokyo is UTC+9. To convert a time, the calculator does three steps:
1. UTC time = time in the first place - its offset 2. time in the second place = UTC time + its offset 3. if the result is before 00:00 or after 24:00, the day changes
The offset of a place is not always the same. In summer, many places move their clocks forward by one hour, so the offset grows by one hour. This is daylight saving time. The calculator looks up the exact rule for the date that you enter, from the time zone data that is built into your browser, so you do not have to know who has changed their clocks.
A worked example
It is 12:00 in London on 10 January. What time is it in New York?
| Step | Result |
|---|---|
| London offset in January (winter time) | UTC+0 |
| UTC time | 12:00 |
| New York offset in January | UTC-5 |
| Time in New York | 07:00, the same day |
Now the same question on 20 March. The United States moved its clocks forward on 8 March, but the United Kingdom does this on 29 March. So for three weeks the difference is one hour less:
| Date | London offset | New York offset | 12:00 in London is |
|---|---|---|---|
| 10 January | UTC+0 | UTC-5 | 07:00 in New York |
| 20 March | UTC+0 | UTC-4 | 08:00 in New York |
| 30 March | UTC+1 | UTC-4 | 07:00 in New York |
This is the reason why the date is needed. A calculator that ignores it will be an hour wrong for part of every year.
The same moment around the world
What time is it in each place when it is 12:00 UTC?
| Place | In January | In July |
|---|---|---|
| New York | 07:00 | 08:00 |
| Los Angeles | 04:00 | 05:00 |
| London | 12:00 | 13:00 |
| Paris | 13:00 | 14:00 |
| Dubai | 16:00 | 16:00 |
| Kolkata (India) | 17:30 | 17:30 |
| Tokyo | 21:00 | 21:00 |
| Sydney | 23:00 | 22:00 |
Dubai, India and Japan do not change their clocks. Sydney is in the southern hemisphere, so its summer is in January, and its clocks are one hour ahead then. India is on a half-hour offset, UTC+5:30.
Chart: what time it is in each place when it is 12:00 UTC (January)
Green is working hours (9:00 to 17:00), amber is early morning or evening, and dark grey is night. Each block is about 36 minutes.
New York ████████████ 07:00 Los Angeles ███████ 04:00 London ████████████████████ 12:00 Paris ██████████████████████ 13:00 Dubai ███████████████████████████ 16:00 Kolkata █████████████████████████████ 17:30 Tokyo ███████████████████████████████████ 21:00 Sydney ██████████████████████████████████████ 23:00
Finding a time that works for both
The calculator also shows how many hours of a working day, from 9:00 to 17:00, the two places have at the same time. This is the window for a live meeting. For some pairs of places, in January:
| Between | Time difference | Shared working hours |
|---|---|---|
| London and Paris | 1 hour | 7 hours |
| New York and Los Angeles | 3 hours | 5 hours |
| London and Dubai | 4 hours | 4 hours |
| London and New York | 5 hours | 3 hours |
| London and Kolkata | 5.5 hours | 2.5 hours |
| New York and Paris | 6 hours | 2 hours |
| Los Angeles and Tokyo | 17 hours | 1 hour |
| London and Tokyo | 9 hours | 0 hours |
| New York and Tokyo | 14 hours | 0 hours |
London - Paris ███████████████████████████████████ 7 h New York - Los Angeles █████████████████████████ 5 h London - Dubai ████████████████████ 4 h London - New York ███████████████ 3 h London - Kolkata █████████████ 2.5 h New York - Paris ██████████ 2 h Los Angeles - Tokyo █████ 1 h London - Tokyo 0 h New York - Tokyo 0 h
The chart in the calculator shows one day in the first place, hour by hour. The working hours of the first place are green, and the working hours of the second place are blue. Where they overlap, both places are at work. The dark line shows your time.
When the clocks change
Two odd things can happen on the day when the clocks change, and the calculator tells you when they do:
- A time that does not exist. In spring the clocks jump forward. In New York on 8 March 2026, the clocks go from 01:59 straight to 03:00, so 02:30 never happens. The calculator uses 03:30, the time that the clocks show right after the change.
- A time that happens twice. In autumn the clocks go back. In New York on 1 November 2026, the hour from 01:00 to 02:00 happens twice. The calculator uses the first time, and says so.
Tips
- Always check the date, especially in March, April, October and November, when different countries change their clocks on different days.
- Say the zone with the time, for example "3 PM Eastern" or "15:00 UTC", when you invite people.
- Use UTC for events around the world. It never changes for daylight saving time.
- Watch out for the date line. Places that are far apart, such as Tokyo and Los Angeles, can be on different days.
- Remember the southern hemisphere. Sydney and other places there change their clocks at the opposite time of the year.
What this calculator does not include
- Places that are not in the list, such as small islands and some regions.
- Rules in the past or the future that are different from your browser's data. The rules can change when a government changes them, and older devices may not know.
- Dates before 1970 and after 2100.
- Leap seconds, which are not counted in normal clocks.
This tool uses the time zone data of your browser and is for general information. For a flight, a contract or a legal deadline, confirm the time with an official source.