Use the Time Zones Explained UTC GMT and Daylight Saving
A time zone is a local time rule used by a country, region or city. It usually has an offset from UTC, such as UTC+1 or UTC-5, and may also change during daylight saving periods.
The UK uses Greenwich Mean Time in winter and British Summer Time in summer. In practical terms, London is UTC+0 during GMT and UTC+1 during BST.
The safest way to compare times is to convert the local time into a single instant, then display that same instant in the destination location. That is what a good time-zone converter does.
Need to convert a meeting time?
Use the time zone converter to compare UK, US, Europe and Asia times with same-day, previous-day and next-day notes.
UTC, GMT and BST in plain English
GMT stands for Greenwich Mean Time. It is the UK's standard winter time and is the time most people mean when they say “UK time” outside the summer months.
UTC stands for Coordinated Universal Time. It is the reference point used for modern timekeeping, software, calendars, servers and international conversion. For everyday UK winter use, UTC and GMT usually show the same clock time, but they are not quite the same concept.
BST stands for British Summer Time. During BST, the UK clock is one hour ahead of GMT, so London is UTC+1.
| Term | What it means | UK offset |
|---|---|---|
| UTC | Global reference time used for conversions and systems. | Reference point |
| GMT | Greenwich Mean Time, used as UK standard time in winter. | UTC+0 |
| BST | British Summer Time, used in the UK during summer time. | UTC+1 |
How daylight saving changes UK time
Daylight saving time is the practice of moving clocks forward for part of the year. In the UK, clocks go forward by one hour in spring and back by one hour in autumn.
GOV.UK says UK clocks go forward at 1am on the last Sunday in March and go back at 2am on the last Sunday in October. In 2026, the clocks went forward on 29 March and go back on 25 October.
How time-zone conversion works
Time-zone conversion is easiest if you think in three steps. First, take the source local time. Second, convert it to UTC. Third, display that same instant in the destination time zone.
Source local time → UTC instant → destination local time
Example:
London 09:00 in June 2026 = UTC 08:00
New York in June 2026 = UTC-4
Converted time = New York 04:00The result can fall on the previous day, the same day or the next day. This is why date as well as time matters when arranging international calls or deadlines.
Worked examples
London to New York in summer
On 20 June 2026, London is on BST, which is UTC+1. New York is on Eastern Daylight Time, which is UTC-4. A 9:00am London meeting is therefore 4:00am in New York.
London to New York in winter
On 20 December 2026, London is on GMT, which is UTC+0. New York is usually UTC-5 in winter. A 9:00am London meeting is again 4:00am in New York, but the offset relationship is different.
London to Tokyo late at night
If it is 10:00pm in London during BST, Tokyo is usually eight hours ahead of London. That makes the converted time 6:00am the next day in Tokyo.
Common time-zone mistakes
- Writing “GMT” when you mean current UK time in summer.
- Forgetting that a converted time can land on the next or previous date.
- Using a fixed offset, such as UTC+1, without checking daylight saving.
- Assuming every country changes clocks on the same date.
- Not including the date when sharing a time-zone conversion.
For international meetings, the clearest format is usually: date, local time, time zone abbreviation and converted time for the main audience.
Best way to share an international time
A good meeting-time message removes ambiguity. Instead of saying “Friday at 3”, include the full date, time zone and one or two converted examples.
Time zones, countdowns and deadlines
Time zones matter most when a deadline is tied to a specific location. “Midnight on Friday” could mean the start or end of Friday, and it could be a different moment in another country.
For events and launches, use a fixed local time and location first, then convert it for other audiences. For a live event, use the countdown timer after the final time zone is agreed.
FAQs
Is the UK always on GMT?
No. The UK uses GMT in winter and BST in summer. During BST, UK local time is one hour ahead of GMT.
Is UTC the same as GMT?
They often show the same clock time, but they are not exactly the same thing. UTC is the modern international reference time used for timekeeping and conversion, while GMT is a time zone/name linked to Greenwich.
Why did my converted time move to the next day?
Because the destination time zone may be many hours ahead of the source time zone. Late evening in one country can already be the next morning elsewhere.
Do all countries change clocks at the same time?
No. Some countries do not use daylight saving at all, and countries that do use it may change clocks on different dates.
Should I write GMT or London time?
For everyday planning, “London time” plus the date is often safer. If you write GMT in summer, people may read it as UTC+0 even though London is using BST.
Sources and checks
- GOV.UK clock-change guidance for UK spring and autumn clock changes.
- National Physical Laboratory explanation of UK clock changes and UTC timing.
- Timeanddate.com explanations of GMT, UTC, BST and daylight saving handling.