Convert GMT to UTC Greenwich Mean Time ↔ Coordinated Universal Time Free Online Secure
Greenwich Mean Time and Coordinated Universal Time read the same on the clock — the offset is zero. Type a time in GMT and it comes back unchanged in UTC, which is the honest answer; what differs between the two names is explained below.
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Through the day — Convert GMT to UTC (Greenwich Mean Time ↔ Coordinated Universal Time)
| GMT | UTC |
|---|---|
| 00:00 | 00:00 |
| 03:00 | 03:00 |
| 06:00 | 06:00 |
| 09:00 | 09:00 |
| 12:00 | 12:00 |
| 15:00 | 15:00 |
| 18:00 | 18:00 |
| 21:00 | 21:00 |
Where these units come from
Until the middle of the nineteenth century every town kept its own noon, set by the sun overhead, and nobody minded that the next town was four minutes behind. Railways ended that: a timetable cannot be written for a hundred local noons. Britain put its railways on Greenwich time in 1847, and in 1884 the International Meridian Conference in Washington chose Greenwich as the prime meridian and sketched the twenty-four-hour scheme we still use. Greenwich Mean Time was the world's reference until 1972, when Coordinated Universal Time replaced it: UTC is kept by atomic clocks and tied back to the turning Earth by leap seconds. The abbreviations on this page name standard time — the winter setting — which is why each one is a fixed number of hours rather than something that drifts through the year.
Where you meet them
A time zone becomes real the moment two people in different places have to agree on one moment: a call between offices, a flight that lands before it took off, a market that opens while you are asleep, a deadline written in somebody else's afternoon. Server logs, train and flight schedules and anything stamped with a time are kept in UTC precisely so that the question can always be settled, no matter whose clock was on the wall.
Questions people ask
What about summer time?
The offsets here are standard-time offsets, the ones a zone keeps in winter. Where summer time is observed, the same places move one hour further on for part of the year and take a different name: Central European Time becomes Central European Summer Time, Eastern Standard Time becomes Eastern Daylight Time. The switching dates are not the same in every country, and the two hemispheres change in opposite months, so for a date inside a summer-time period add an hour to whichever side observes it.
Why isn't every difference a whole number of hours?
Because offsets are decided by governments, not by geometry. India is half an hour off the hour, at UTC+05:30; Nepal is three quarters, at UTC+05:45; the Chatham Islands are at UTC+12:45. Any of these can be changed by law, and several countries have moved their offset within living memory.
Is the answer exact?
Where both units are defined in terms of one another by whole numbers, yes — the page says so above the figure, and the only rounding is in how many digits are shown. Where the ratio has no finite decimal, the page shows twelve significant digits and says the relation is approximate rather than pretending otherwise.
Does anything I type leave my device?
No. The arithmetic runs in your browser; nothing is uploaded and there is no server to upload it to. Load the page once and it keeps working with the network switched off.
When an auditor asks for UTC and your notes say GMT
It happens in compliance reviews, in API documentation, and in handover notes between teams. Somebody wrote “maintenance window: 22:00 to 23:30 GMT”, and somebody else needs the same window written in UTC. If you are asked to convert GMT to UTC, the honest answer is that the numbers stay put. 22:00 GMT is 22:00 UTC, and 23:30 GMT is 23:30 UTC. The offset between the two scales is zero, and the date never shifts.
That does not make the question silly. The two names have different histories, different owners and different quirks in software, and a careless rewrite can introduce errors. This page gives you the rule, a set of worked readings, the stories behind each name, the places where software mislabels them, and a short list of rules for writing times that nobody will misread. All arithmetic takes place in your browser.
The conversion step by step
What to write
Read the hours and minutes on the GMT side and write the identical hours and minutes on the UTC side. When you convert GMT to UTC you add zero minutes. The interval 22:00 to 23:30 on the GMT side is therefore 22:00 to 23:30 on the UTC side, and an interval that crosses midnight keeps crossing it in exactly the same place.
Readings to compare against
01:10 GMT is 01:10 UTC. 05:50 GMT is 05:50 UTC. 10:25 GMT is 10:25 UTC. 14:00 GMT is 14:00 UTC. 17:35 GMT is 17:35 UTC. 22:00 GMT is 22:00 UTC. If the box above shows anything else for any of these, something other than this conversion is going on.
What still needs a second step
The question “what is that on my watch?” needs a different offset. Someone in Bucharest adds two hours in winter and three in summer. Someone in Chicago subtracts six in winter and five in summer. The conversion on this page stops at the shared reference and leaves that step to you.
The story of GMT
From sea charts to railway timetables
The Longitude Act of 1714 offered a large prize for a reliable way to find a ship’s position, and the winning idea was a clock that kept the time of a home port while the ship sailed. John Harrison’s marine chronometers, tested in the early 1760s, made this practical, and for British ships the home time was the time at Greenwich. Railways later needed one agreed time for the whole country, and by the 1840s most British lines ran on Greenwich time. A law of 1880 made it official.
The world agrees on a meridian
At the International Meridian Conference in Washington in 1884, twenty-two of twenty-five delegations voted for Greenwich as the prime meridian. San Domingo voted against, and France and Brazil abstained. GMT became the common reference for sea charts and for international telegraph schedules for most of the next century.
Today
The Royal Observatory still stands in Greenwich Park, and visitors photograph the brass line there, although working astronomy moved away in the 1950s. GMT survives as the legal winter time of Britain and Ireland, the permanent clock of Iceland and several West African countries, and a label inside many protocols.
The story of UTC
Why a new name was needed
By the 1950s the first atomic clocks showed that Earth’s rotation is not perfectly steady, so a time scale based on the Sun could not serve physics, radio and navigation. In 1960 the first system that combined atomic seconds with the rotation of the Earth appeared, and the present form of UTC began in 1972. Its abbreviation was agreed as a compromise between English and French word orders.
Who keeps it
The International Bureau of Weights and Measures near Paris combines readings from clocks in dozens of national laboratories. Each country then steers its own version of UTC to stay within a few billionths of a second of the combined value. The result reaches the public through radio signals, satellite navigation systems and network time servers.
What it does for you
UTC is the single column in which logs, databases and flight plans line up. Whenever you convert GMT to UTC, you are moving from a historical name to the technical one, without disturbing the reading.
Reading GMT and UTC side by side
The practical equivalence
The two scales stay within one second of one another by design, so for any task that involves scheduling, logging or conversation the offset is zero. Dictionaries, standards bodies and operating systems list GMT as a synonym for UTC for exactly this reason.
Where the vocabulary diverges
People who write specifications prefer UTC because it names a measured standard. Journalists and travellers prefer GMT because it is the older and friendlier word. Neither choice changes the hour. When your destination document asks for UTC, replace the label and leave the numbers alone: that is the whole job when you convert GMT to UTC.
The one difference that costs money
A person who says “GMT” in July is often talking about the clock in London, which is an hour ahead of the real GMT. Treat that as a statement about London, not about GMT, and convert it separately.
Summer time, and who observes it
The British and Irish calendar
Clocks in the United Kingdom go forward at 01:00 UTC on the last Sunday in March and back at 01:00 UTC on the last Sunday in October. In 2026 that means 29 March and 25 October. Ireland follows the same dates. Between them, the label on the wall clock is British Summer Time in Britain and Irish Standard Time in Ireland, both UTC+1.
An odd corner of Irish law
Irish law defines summer as standard time and treats the winter clock as GMT. The time zone database represents this as a negative daylight saving offset in winter. For you the effect is the same: Dublin is UTC+0 in winter and UTC+1 in summer, like London.
What follows for the conversion
Nothing changes for GMT and UTC themselves, and you can convert GMT to UTC on any date of the year. Only a statement about a British or Irish wall clock needs checking against the calendar.
How software names the zones
Windows
The Windows time zone list contains three similar entries. “UTC” is a fixed zero offset. “Greenwich Standard Time”, used for Monrovia and Reykjavik, is also fixed at zero. “GMT Standard Time”, used for London, Dublin, Edinburgh and Lisbon, follows summer time. A server set to the third will move an hour twice a year, which surprises people who chose it expecting GMT all year.
Unix and the zone database
On Linux and macOS the names UTC, GMT, Etc/UTC, Etc/GMT and Zulu are all fixed at zero. Real places have their own entries: Europe/London, Europe/Dublin, Atlantic/Reykjavik. For a server that logs events, pick UTC and keep it. For a person who needs the local view, pick a city.
Java, JavaScript and Python
How each language treats the pair
Java offers ZoneOffset.UTC and ZoneId.of("GMT"), which behave identically for arithmetic. JavaScript prints UTC dates with the suffix GMT in toUTCString() and with the letter Z in toISOString(). Python’s timezone.utc and ZoneInfo("GMT") both give a fixed zero offset. A routine that must convert GMT to UTC therefore reduces to returning the input unchanged.
Do not hard-code the label
If your code stores times, store them as instants, not strings. Then no one needs to choose between GMT and UTC when displaying them, and your users see whatever label the interface prefers.
Ships’ logs and the sea
A tradition of Greenwich time
Ships have kept logbooks in Greenwich time since the age of chronometers, because it gave every vessel a shared reference that did not depend on which ocean it sailed. Modern bridge systems and distress messages record UTC instead, while navigators still say GMT in conversation.
Why it still matters on shore
A shipping schedule that quotes “ETA 06:00 GMT” and a port system that expects 06:00 UTC are describing the same instant. When you convert GMT to UTC, you can pass the figure straight through, then add a local offset for the harbour office.
Weather reports
Synoptic hours
Weather services collect observations at fixed hours of the day, traditionally called 00, 06, 12 and 18 GMT, and now called UTC. A chart labelled “12Z” is built from observations at 12:00 on that scale. The forecast for the region you live in might be released a little later, but its reference time is the same worldwide.
Local interpretation
In Romania, 12 UTC is 14:00 in winter and 15:00 in summer. In London, 12 UTC is 12:00 in winter and 13:00 in summer. In Chicago, it is 06:00 in winter and 07:00 in summer. Nothing in the label tells you which of these applies, so keep the date with the chart.
The Greenwich time ball and the pips
Two ways the public learned the time
Since 1833 a red ball has dropped down a mast at the Royal Observatory at one o’clock, so ships on the Thames could set their chronometers. In 1924 the BBC began to broadcast six short tones, the Greenwich Time Signal, to the entire country. Both were once simple GMT services. Today the radio signal follows the clock in force in Britain, which is GMT in winter and BST in summer.
Lessons for modern writing
The signals taught millions of people to equate the word Greenwich with the time on their kitchen clock, and that habit is why “GMT” still appears in messages that really mean “London time”.
Documents, standards and style guides
What style guides say
Technical style guides tell authors to write UTC, to give dates in the order year-month-day and to avoid ambiguous terms such as “midnight” or “noon” unless the date is also given. The advice follows from ISO 8601, which also defines the notation 2026-10-13T22:00:00Z.
Rewriting an old document
In an older manual, search for GMT, check that each mention is about a machine or protocol rather than about London, then replace the label with UTC. The numbers stay the same. When you convert GMT to UTC across an entire handbook, that rule covers nearly every line.
Conferences, webinars and launches
Publish one anchor time
An organiser inviting people worldwide should publish a single anchor, such as “Tuesday 13 October, 15:00 UTC”, and leave the local equivalents to a converter or a calendar file. That avoids the scene in which half the audience arrives an hour early because the organiser was thinking about London’s summer clock.
Local equivalents on the same page
A short table helps: Bucharest 18:00 in October before the clocks change on 25 October, Chicago 10:00, Mumbai 20:30, Tokyo 00:00 the next day. Computing each from UTC avoids chains of conversions, and chains are where mistakes breed.
Airports and the flight schedule
The pilot’s clock
Cockpits and control towers work in UTC, which pilots call Zulu. Flight plans, forecasts and notices to airmen are all stamped that way. A line such as “ETD 1830Z” means 18:30 UTC, which for a traveller means 20:30 in Bucharest in winter or 21:30 in summer.
The ticket in your hand
Boarding passes list local times, so a pilot’s document and your document use different clocks. Comparing them needs the airport’s local offset on that date. The step from GMT to UTC adds nothing to that effort, so you can convert GMT to UTC first and worry about the airport afterwards.
Countries that do not change their clocks
The all-year GMT list
Iceland keeps UTC+0 all year, despite sitting so far west that the Sun is highest well after one o’clock. Ghana, Senegal, Mali, Guinea, Sierra Leone, Liberia and Côte d’Ivoire do the same. In these places a person who says GMT is right at every time of year.
Countries that moved away
Some countries on the western side of Europe and Africa have changed zones in the past century as political choices changed. Portugal has switched between zero and one hour ahead more than once, and Spain moved to Central European time in 1940. When you handle old data, check the history of the specific place.
A quick audit of your own notes
Three questions
Did the source describe a machine or a person? Does the date fall between the last Sunday in March and the last Sunday in October? Does your destination require a label, a numeric offset, or a city? Answer these and you will know whether to convert GMT to UTC by copying or by adjusting.
If in doubt, write the offset
A notation such as 22:00+00:00 states the offset itself, and no one can dispute it. It is also accepted by every date parser you are likely to meet.
Mixed teams across the Atlantic
Why the gap is not constant
The United States starts summer time on the second Sunday of March and ends it on the first Sunday of November. Europe starts on the last Sunday of March and ends on the last Sunday of October. In 2026 the US moves on 8 March, and Europe follows on 29 March. For three weeks the gap between London and New York is four hours instead of five.
Why UTC is the neutral party
Both sides can describe the same event in UTC. After you convert GMT to UTC, each side adds its own offset, and none of them depends on what the other has done with its clocks.
Support desks and the follow-the-sun model
One rota, many offices
A global help desk hands tickets from office to office as the day moves around the planet. The rota is written once in UTC and displayed in each office’s local time. Because UTC never moves, managers can edit the rota in January and be confident it still works in July, even though local clocks have shifted.
An example
An Australian shift ends at 06:00 UTC, a Romanian shift runs from 06:00 to 14:00 UTC, and a Texan shift takes over at 14:00 UTC. Locally, the Romanian shift is 08:00 to 16:00 in winter and 09:00 to 17:00 in summer. A team that writes the rota as GMT instead of UTC gives the same hours, and a quick convert GMT to UTC confirms it.
Checking your own conversion by hand
Two minutes, no tools
Take any single reading, say 17:35 GMT. Write it down in the form 17:35 UTC. Hold both up against a trusted clock set to zero offset. They match. Then ask what the date was in London, and whether the Londoner meant the wall clock or the zero-offset scale. That second question carries all the risk.
When to stop checking
If the source is a log, a protocol, a certificate or a scientific schedule, you can stop after the first step. If the source is a person talking about London in summer, convert separately.
Checklist: how to convert GMT to UTC safely
Before
Identify the source
Machine, protocol or person? That one fact decides everything.
Identify the destination
If it asks for UTC, copy the reading. If it asks for local time, plan a second step.
After
Check the result
The hours, minutes and date should match the input. Any difference signals a different conversion.
Record it
Write the date, the reading and the label UTC. In public documents add the local equivalent.
Mistakes people make when they convert GMT to UTC
Subtracting an hour in summer
The urge is strong because London is an hour ahead in July. But the offset between GMT and UTC is zero in July as in January.
Choosing GMT Standard Time on a server
This Windows entry follows summer time. It is not the fixed zero zone. If you want a steady clock, pick UTC.
Reading Etc/GMT+3 as three hours ahead
The sign is inverted in that naming scheme, and Etc/GMT+3 is three hours behind UTC.
Writing times without dates
A time with no date cannot be verified against the calendar, and so it may be wrong by an hour for people on British summer time.
Questions about GMT and UTC
What do I add when I convert GMT to UTC?
Nothing. The offset is zero hours.
Why do standards ask for UTC?
Because UTC is the measured, internationally maintained scale, and because it avoids the summer-time ambiguity of London.
Is GMT Standard Time the same as UTC?
No. In Windows it follows British summer time, so it is not fixed.
Can the date change?
No. The conversion keeps the date.
Is my input sent to a server?
No. Everything runs in your browser.
More zone conversions
Other converters worth a look: Convert GMT to PST, Convert GMT to CST, Convert CST to UTC, Convert WET to UTC, Convert UTC to CET, Convert CET to GMT, Convert CST to PST, Convert EST to UTC, Convert GMT to CET. For the opposite direction, see Convert UTC to GMT.
To convert GMT to UTC, then, you copy the reading and change the label. Keep the London summer clock in a separate box in your mind, and the whole subject reduces to one rule. Calculation is local, free, and requires no account.