What is the difference between GMT and UTC?

GMT is a time zone; UTC is a time standard. Greenwich Mean Time is the local civil time of the Prime Meridian, used by the UK in winter. Coordinated Universal Time is the atomic reference every zone is defined against and never changes for daylight saving. In practice they read the same in the British winter, but in July the UK is on BST and therefore an hour ahead of UTC.

How many time zones are there?

There are 38 distinct UTC offsets in current use, spanning UTC−12 to UTC+14. The count is higher than 24 because several zones use half-hour or 45-minute offsets, and because a few Pacific territories sit beyond the ±12 boundary.

Why does China have only one time zone?

China spans roughly five hours of solar time but keeps a single zone at UTC+8 for administrative unity. The practical result is that in western Xinjiang the sun can rise after 10:00am by the official clock, and many residents keep an unofficial local time alongside it.

Time zones explained

What the abbreviations mean, which countries use them, and when they shift.

Common time zones

Ordered west to east by standard offset.

HSTUTC−10
Hawaii Standard Time
Ten hours behind UTC with no daylight saving, making Hawaii one of the furthest-behind inhabited places on earth. It is eleven hours behind Wales in the British summer.
AKSTUTC−9
Alaska Standard Time
Nine hours behind UTC, moving to AKDT (UTC−8) in summer. Anchorage sees under six hours of daylight at the winter solstice and more than nineteen at midsummer.
PSTUTC−8
Pacific Standard Time
Eight hours behind UTC on the US west coast, and the zone most software release schedules are quoted in. Becomes Pacific Daylight Time (UTC−7) from March to November.
MSTUTC−7
Mountain Standard Time
Seven hours behind UTC across the US Mountain states. Arizona keeps MST all year and does not observe daylight saving, which puts it on the same clock as California in summer.
CSTUTC−6
Central Standard Time
Six hours behind UTC, covering the central United States, much of Mexico and parts of Central America. Becomes Central Daylight Time (UTC−5) in summer.
ESTUTC−5
Eastern Standard Time
The winter offset of the US east coast, five hours behind UTC. It becomes Eastern Daylight Time (UTC−4) from the second Sunday in March to the first Sunday in November.
BRTUTC−3
Brasília Time
Three hours behind UTC across Brazil’s populous south-east. Brazil abolished daylight saving in 2019, so the offset is now fixed year-round.
GMTUTC
Greenwich Mean Time
The time standard at the Prime Meridian, and the baseline every other zone is measured from. The United Kingdom keeps GMT in winter and switches to British Summer Time from late March to late October.
UTCUTC
Coordinated Universal Time
The atomic time standard the world synchronises to. UTC never changes for daylight saving, which is why aviation, computing and science use it in preference to a local zone. In winter it matches GMT exactly.
BSTUTC+1
British Summer Time
The United Kingdom’s summer offset, one hour ahead of GMT. It runs from 1:00am on the last Sunday in March to 2:00am on the last Sunday in October, covering Wales, England, Scotland and Northern Ireland.
CETUTC+1
Central European Time
The winter offset for most of mainland western and central Europe, one hour ahead of the UK. It becomes Central European Summer Time (UTC+2) between late March and late October.
EETUTC+2
Eastern European Time
Two hours ahead of UTC, covering the eastern edge of Europe and parts of the Middle East and North Africa. It moves to Eastern European Summer Time (UTC+3) in summer.
CESTUTC+2
Central European Summer Time
The summer offset for mainland western Europe, two hours ahead of UTC and one hour ahead of British Summer Time. It runs from the last Sunday in March to the last Sunday in October.
SASTUTC+2
South Africa Standard Time
Two hours ahead of UTC with no daylight saving, covering South Africa, Lesotho and Eswatini. In the British summer the gap to Wales is just one hour.
MSKUTC+3
Moscow Time
Three hours ahead of UTC. Russia abolished seasonal clock changes in 2014, so Moscow Time is now permanently fixed and the gap to the UK shifts with British Summer Time instead.
GSTUTC+4
Gulf Standard Time
Four hours ahead of UTC across the Arabian Gulf. None of the Gulf states observe daylight saving, so the offset from the UK varies between three and four hours depending on the British season.
PKTUTC+5
Pakistan Standard Time
Five hours ahead of UTC with no daylight saving, thirty minutes behind neighbouring India.
ISTUTC+5:30
India Standard Time
Five and a half hours ahead of UTC, used across the whole of India regardless of how far east or west a city lies. India has never observed daylight saving, so the offset is constant all year.
SGTUTC+8
Singapore Time
Eight hours ahead of UTC. Singapore sits almost on the equator but keeps a zone more than an hour ahead of its solar time, a legacy of aligning with Malaysia in 1982.
HKTUTC+8
Hong Kong Time
Eight hours ahead of UTC, shared with mainland China, Taiwan and the Philippines. Hong Kong abandoned daylight saving in 1979.
CSTUTC+8
China Standard Time
A single zone eight hours ahead of UTC covering the whole of China, despite the country spanning roughly five hours of solar time. Sunrise in western Xinjiang can therefore fall after 10:00am on the clock.
JSTUTC+9
Japan Standard Time
Nine hours ahead of UTC across the whole of Japan. Japan has not used daylight saving since a brief post-war experiment, so the offset never changes.
AESTUTC+10
Australian Eastern Standard Time
Ten hours ahead of UTC on Australia’s east coast. New South Wales, Victoria, Tasmania and the ACT move to AEDT (UTC+11) in the southern summer, but Queensland stays on AEST all year.
NZSTUTC+12
New Zealand Standard Time
Twelve hours ahead of UTC, moving to NZDT (UTC+13) from late September to early April. New Zealand is among the first countries to enter each new day.

How time zones came to exist

Before the railways, every town kept its own solar noon. Bristol ran about ten minutes behind London, and nobody minded until timetables made the discrepancy dangerous. Britain standardised on Greenwich time through the 1840s, and in 1884 the International Meridian Conference in Washington fixed Greenwich as the Prime Meridian for the world.

The modern system divides the globe into offsets from Coordinated Universal Time, but the boundaries follow politics rather than geography. Offsets are maintained in the IANA time zone database, which records not just current rules but every historical change which is why software can correctly tell you what the clock read in Cardiff on a given morning in 1973.

Time zones, questions

What is the difference between GMT and UTC?: GMT is a time zone; UTC is a time standard. Greenwich Mean Time is the local civil time of the Prime Meridian, used by the UK in winter. Coordinated Universal Time is the atomic reference every zone is defined against and never changes for daylight saving. In practice they read the same in the British winter, but in July the UK is on BST and therefore an hour ahead of UTC.
How many time zones are there?: There are 38 distinct UTC offsets in current use, spanning UTC−12 to UTC+14. The count is higher than 24 because several zones use half-hour or 45-minute offsets, and because a few Pacific territories sit beyond the ±12 boundary.
Why does China have only one time zone?: China spans roughly five hours of solar time but keeps a single zone at UTC+8 for administrative unity. The practical result is that in western Xinjiang the sun can rise after 10:00am by the official clock, and many residents keep an unofficial local time alongside it.