To convert a time between two places, turn it into UTC, then add or subtract the other place’s offset for that date. London at 09:00 on 15 July 2026 is 08:00 UTC, which is 04:00 in New York, five hours earlier. The offset can change with the season, so always convert for the specific date, not just “usually”.
How time zones work
The world is divided into zones, each with an offset from Coordinated Universal Time (UTC). A zone that is “UTC+1” is one hour ahead of UTC, and “UTC-5” is five hours behind. Most offsets are whole hours, but some are not: India is 5 hours 30 minutes ahead of UTC.
Because the offset is the only thing that matters, the safest method is always the same:
- Write the local time and date.
- Subtract that place’s offset to get UTC.
- Add the offset of the other place.
- Check whether the result has crossed midnight, which changes the date.
UTC, GMT and BST
GMT and UTC are the same for everyday purposes. The UK uses GMT (UTC+0) in winter and British Summer Time, BST (UTC+1), in summer. Clocks go forward on the last Sunday of March and back on the last Sunday of October. So “UK time” is not one fixed offset, and a meeting that is “10:00 UK time” in January is at a different UTC time from the same meeting in July.
Why daylight saving causes mistakes
The biggest source of error is that countries change their clocks on different dates. The US changes on the second Sunday of March and the first Sunday of November, while the UK changes on the last Sunday of March and the last Sunday of October.
That leaves short windows each year when the usual gap between the two countries is wrong by an hour. Here is the same 09:00 London time converted for three dates, using the Time zone converter tool:
| London time | Date | New York time | Gap |
|---|---|---|---|
| 09:00 | Monday 23 March 2026 | 05:00 | 4 hours |
| 09:00 | Monday 30 March 2026 | 04:00 | 5 hours |
| 09:00 | Wednesday 15 July 2026 | 04:00 | 5 hours |
On 23 March the US had already moved its clocks forward but the UK had not, so the gap was four hours. A week later both had changed and it returned to five. If you are not certain about another country’s rules, check an official source or use a tool that handles the date for you.
A worked example: a call between London and Kolkata
Suppose you want a call at 09:00 in London on 15 July 2026. London is on BST, so that is 08:00 UTC. India is UTC+5:30, so the time in Kolkata is 13:30. In winter, London is on GMT, so 09:00 is 09:00 UTC and the same call falls at 14:30 in India. The meeting time is the same on the clock in London, but the gap between the two places differs by an hour.
Tips for scheduling across time zones
- Always state the time zone and the date when you invite someone, for example “14:00 UK time on Tuesday 14 July”.
- Be careful with calls near midnight, where the date differs between the two places.
- Check for daylight saving changes in the weeks around March, April, October and November.
- If you travel, remember flight times on tickets are local times. The guide on How flight times are worked out explains how to work out the actual duration.
- If you plan work across countries, remember that public holidays also differ, which the guide to UK bank holidays and how to count working days covers for the UK.
Note: This guide is general information. Clock-change rules are set by governments and can change, so check an official source for anything important, such as a legal deadline or a flight booking.
Try the calculator
Pick a date, a time and two cities in the Time zone converter converter and it shows the converted time, the difference and the same moment in other major cities. Daylight saving is included for the date you choose.
Open the free Time zone converter
Frequently asked questions
What is the difference between GMT and UTC?
For everyday use they are the same time. UTC is the modern standard that clocks are set against, while GMT is the name used in the UK for its winter time.
Does the UK time difference to the US stay the same all year?
Usually it is five hours to New York, but it can be four hours for a few weeks in spring and autumn because the two countries change their clocks on different dates.
Why is India half an hour off?
India uses a single offset of UTC+5:30, so it is not a whole number of hours from London. The gap with the UK is 5 hours 30 minutes in winter and 4 hours 30 minutes in summer.
Should I use the time zone converter for a flight?
For the clock times, yes. To find how long a flight takes, subtract the two local times after allowing for the offset, or use the flight duration tools linked in our flight guide.