GMT, BST and UTC: UK time and time zones explained

Anyone who has scheduled a call with a colleague in another country knows how easily a one-hour mistake creeps in. The UK has two clock times, one for winter and one for summer. Other countries change their clocks on different dates, some do not change them at all, and some use offsets of half an hour. This guide explains the difference between GMT, BST and UTC, gives the dates the UK clocks change in 2026 and 2027, shows how to convert a meeting time to several major places with worked examples, and explains the short periods each year when the gap between the UK and the USA is different from normal.

By Valtrinity. Updated 18 October 2026.

GMT, BST and UTC: what the letters mean

GMT is Greenwich Mean Time, the time on the meridian that passes through Greenwich in London. It is the UK's standard time during the winter. BST is British Summer Time, which is one hour ahead of GMT and is used in the summer so that evenings stay lighter for longer.

UTC stands for Coordinated Universal Time. It is the international standard that scientists, pilots and computer systems use as a reference, and it does not change for daylight saving. For everyday purposes UTC and GMT are the same time on the clock, so in winter London is on UTC plus zero, and in summer it is on UTC plus one.

Written as offsets, the UK is on UTC+0 in the winter and UTC+1 in the summer. When another country gives a time in UTC, you add one hour to get the UK time between the end of March and the end of October.

When the UK clocks change

The rule is simple. British Summer Time starts on the last Sunday of March, when the clocks go forward from 1am to 2am. It ends on the last Sunday of October, when the clocks go back from 2am to 1am. The change happens at 1am UTC in both cases.

In 2026, the clocks go forward on Sunday 29 March and back on Sunday 25 October. In 2027, they go forward on Sunday 28 March and back on Sunday 31 October. The spring change loses an hour, so that Sunday is 23 hours long, and the autumn change gains an hour, so that Sunday is 25 hours long.

A rhyme that helps people remember the direction is that the clocks spring forward in spring and fall back in autumn. For devices such as phones and computers, the change is normally automatic, but check any clock that is set by hand, such as a cooker, a car or a timer.

Converting a meeting time: worked examples

Suppose a meeting is at 3pm in London in mid-July, when the UK is on BST. In UTC that is 2pm. New York is on Eastern Daylight Time, which is UTC minus 4, so it is 10am there. Paris and Berlin are on Central European Summer Time, UTC plus 2, so it is 4pm. India is on UTC plus 5:30, so it is 7:30pm. Tokyo is UTC plus 9, so it is 11pm. Sydney is on Australian Eastern Standard Time, UTC plus 10, so it is midnight and the date is the next day.

Now the same 3pm in mid-January, when the UK is on GMT. UTC is 3pm. New York is on Eastern Standard Time, UTC minus 5, so it is 10am. Paris is on UTC plus 1, so it is 4pm. India is still UTC plus 5:30, so it is 8:30pm. Tokyo is midnight. Sydney is on summer time at UTC plus 11, so it is 2am the next day.

Notice that New York is five hours behind London for most of the year, so the gap looks fixed. India's gap changes between 5.5 and 4.5 hours depending on the season, because India does not change its clocks.

The weeks when the gap shifts

The USA changes its clocks on different dates from the UK. In the United States, daylight saving begins on the second Sunday of March and ends on the first Sunday of November. In 2027 that is 14 March and 7 November. The UK changes on 28 March and 31 October.

For two or three weeks in March, and for a week in autumn, the gap between London and New York is four hours instead of five. In 2027, between 14 March and 27 March, 3pm in London is 11am in New York. Between 31 October and 6 November, the gap is four hours again.

European countries in the EU change on the same dates as the UK, the last Sunday of March and October, so the gap with Paris or Berlin is always one hour. Australia changes in the opposite season, because it is in the southern hemisphere, so the gap with Sydney varies by two hours across the year.

If you schedule recurring meetings across countries, check the weeks around the changes, as the invite may move by an hour for one party and not the other.

Writing times clearly

The best way to avoid confusion is to include the zone when you share a time. Writing 3pm London time, or 15:00 UTC, leaves no doubt. Writing 3pm GMT in July is a mistake, because London is not on GMT in July. If you mean the time in the UK, use UK time and let the reader work out whether it is GMT or BST.

The 24-hour clock avoids the confusion between am and pm. In the UK, 12 midnight at the start of a day is often written 00:00 and at the end as 24:00, but 12am is ambiguous. If an event is at midnight, give the date, such as 'midnight at the end of Friday 5 March', or say 23:59 on Friday.

For software and logs, use ISO 8601, which writes the time as 2027-07-14T15:00:00+01:00. The +01:00 shows the offset from UTC, so the time is unambiguous even for people in another country.

Time zones with odd offsets

Most time zones differ from UTC by a whole number of hours, but some have a half or three-quarter hour offset. India is UTC+5:30. Nepal is UTC+5:45. Parts of Australia are on UTC+9:30 and the Chatham Islands in New Zealand are on UTC+12:45.

China uses a single time zone, UTC+8, across the whole country, even though it is wide enough to span five. Russia has eleven zones. The USA has several, with the main ones being Eastern (New York), Central (Chicago), Mountain (Denver) and Pacific (Los Angeles), which are UTC minus 5, 6, 7 and 8 in winter.

When you plan a call with several people, pick a time that works for most, and rotate the inconvenient slot among the group so that the same person is not always working at midnight.

Checking before an important deadline

For a deadline that depends on a precise moment, such as a tender, a ticket sale or a flight, find out which time zone the organiser uses and convert it to your own. Do not assume that midnight means midnight where you are.

On the days the UK clocks change, take care with anything that runs overnight. A task scheduled for 1:30am on 28 March 2027 does not exist in UK local time, because the clocks jump from 1am to 2am. A task scheduled for 1:30am on 31 October 2027 happens twice, once on BST and once on GMT.

A time zone converter does the sums for you, and also knows the dates when each place changes. Use one for anything important, and double-check with a second source if the stakes are high.

Tools mentioned in this guide

Frequently asked questions