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
Worked example 2: the autumn gap between London and New York
The spring mismatch is well known, but the autumn one catches just as many people. In 2026 the UK clocks go back on the last Sunday of October, which is 25 October. The US clocks go back on the first Sunday of November, which is 1 November. For the week in between, London is on GMT while New York is still on summer time.
Here is 09:00 London time converted with the time zone converter:
| London time | Date | New York time | Gap |
|---|---|---|---|
| 09:00 | Monday 26 October 2026 | 05:00 | 4 hours |
| 09:00 | Wednesday 28 October 2026 | 05:00 | 4 hours |
| 09:00 | Monday 2 November 2026 | 04:00 | 5 hours |
A weekly call that sits at 09:00 in London and 04:00 in New York for most of the year would land at 05:00 in New York for that one week, and a regular US attendee might be an hour late or an hour early. The simplest fix is to write the date, the time and the city in every invitation, and let calendar software convert it.
Step-by-step: converting any time using UTC
Using UTC as a middle step avoids nearly every error, because it stops you trying to add and subtract two offsets in your head at once.
- Write down the local time, the date and the place.
- Look up that place’s offset for that date, not today’s date. London is UTC+0 in January and UTC+1 in July.
- Subtract the offset from the local time to get UTC.
- Add the destination’s offset for that same date.
- If the answer is below 00:00 or above 23:59, adjust the date by a day.
Example: a team in Los Angeles proposes 16:00 on 15 July 2026. Los Angeles is on summer time then, which is UTC-7, so 16:00 is 23:00 UTC on 15 July. London is UTC+1 in July, so the time in London is 00:00 on Thursday 16 July. The time zone converter confirms this, with the difference shown as eight hours ahead of Los Angeles.
More examples: Asia and Australia
Some city pairs are steadier than others. Here is 09:00 in London converted for a few places on 15 July 2026, using the tool:
| Destination | Local time | Gap from London |
|---|---|---|
| Dubai | 12:00 | 3 hours ahead |
| India (Mumbai, Delhi) | 13:30 | 4 hours 30 minutes ahead |
| Tokyo | 17:00 | 8 hours ahead |
| Sydney | 18:00 | 9 hours ahead |
Sydney is a useful warning. On 15 December 2026, the same 09:00 London call is 20:00 in Sydney, an 11-hour gap, because the two countries are on summer time at different times of the year. If you work with Australia or New Zealand, never reuse the gap you learned in one season for the other. Treat each country’s clock-change dates as its own rule, and check an official source if you are unsure.
Common mistakes to avoid
- Saying “GMT” in summer. UK time in summer is BST. If you write “10:00 GMT” in July you mean a different moment from “10:00 UK time”.
- Using today’s offset for a future date. A booking in three months may fall after a clock change.
- Forgetting the date line. A late-evening call in London can be the next morning in Tokyo or Sydney. State both dates.
- Assuming 12-hour times are clear. “7:00” can mean morning or evening. The 24-hour clock avoids this.
- Trusting a rule of thumb for half-hour zones. India, for example, is 5 hours 30 minutes ahead of UTC, so quoting a whole-hour gap is wrong.
- Ignoring weekends and holidays. A time that works on a Tuesday may land on a weekend in the Middle East or a bank holiday elsewhere. See UK bank holidays and how to count working days for how to count working days.
Glossary of time zone terms
| Term | Meaning |
|---|---|
| UTC | Coordinated Universal Time, the world standard that local clocks are set against |
| GMT | The UK’s winter time, equal to UTC for everyday purposes |
| BST | British Summer Time, UTC+1, used between the last Sunday of March and the last Sunday of October |
| Offset | How far a local clock is ahead of (+) or behind (-) UTC |
| Daylight saving time | Moving clocks forward in summer, which changes the offset for part of the year |
| Date line | The point where the calendar date changes as you travel east or west |
Travel and sleep planning
A time zone converter is also handy before a long trip. Once you have the offset, you can plan when to adjust your sleeping pattern. The Sleep cycles: how to choose a bedtime or wake-up time guide explains how sleep is organised in cycles, which helps you work out a bedtime when your body clock is several hours out. For flights, ticket times are local, so the actual duration is the difference between departure and arrival after allowing for the offset, as How flight times are worked out explains.
When to check an official source
For legal deadlines, court filings, exam start times, financial markets or flight check-in cut-offs, do not rely on a general guide or a quick mental sum. Use the time stated by the organisation involved, and confirm which time zone it refers to. Clock-change rules are set by governments and occasionally change, so a tool or a rule you learned years ago may not be current. If a booking or contract is silent on the time zone, ask for it to be specified in writing.
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.
What is the difference between UTC+0 and UTC-0?
There is no practical difference. Both mean no offset from UTC. You will normally see UTC+0, or just UTC.
When do the clocks change in the UK in 2026?
They go forward on Sunday 29 March 2026 and back on Sunday 25 October 2026, which are the last Sundays of those months.
What is the best time for a call between London and New York?
Early afternoon in London is usually the most comfortable overlap, because it falls in the morning in New York. Check the date first, since the gap is occasionally four hours rather than five.
Why does my calendar show a different time from my invite?
The invite may have been created in another time zone, or your device may be set to a different one. Check the time zone in the event details and in your device settings.