What does GMT stand for? It means Greenwich Mean Time, the time of day measured at the Royal Observatory in Greenwich, a district of London that sits on the prime meridian, the line of zero degrees longitude. For well over a century, the world set its clocks by counting hours ahead of or behind this point.
Today you meet GMT in meeting invites that say "3 p.m. GMT," on flight schedules, in computer time settings that read "GMT-5," and on travel watches with an extra hand. Each use goes back to the same idea: one shared reference clock for the whole planet.
Quick facts
Stands for
Greenwich Mean TimeType
Initialism, said G M TBased on
The prime meridian at Greenwich, LondonModern standard
UTC, which matches GMT for daily useUsed in
Time zone offsets, travel, aviation, international meetingsIn watches
A 24 hour hand that tracks a second time zoneNot to be confused with
BST, British Summer Time, which is GMT+1
What does GMT stand for, and why "mean"?
| Letter | Word | Meaning |
|---|---|---|
| G | Greenwich | The London district where the observatory stands |
| M | Mean | Averaged, because the sun's timing shifts slightly over the year |
| T | Time | Clock time |
The word "mean" is the key. Solar noon, when the sun is highest, drifts a little earlier or later through the year. Astronomers averaged it out to get a steady clock that runs at the same rate every day. That averaged clock, kept at Greenwich, became Greenwich Mean Time. Say it as three letters, "G M T," and note that Greenwich is usually pronounced "GREN itch."
How Greenwich became the starting line
In 1884, delegates from many countries met at the International Meridian Conference in Washington, D.C., and chose the meridian through Greenwich as zero longitude. British sea charts already used it widely, so it was a practical pick. Time zones around the world were then described as whole hours ahead of or behind GMT, which is why New York, Tokyo and Sydney all have offsets measured from London.
GMT vs UTC: are they the same?
For everyday clocks, yes, they show the same time. The difference is how each is kept. GMT came from watching the sun and stars. Coordinated Universal Time, or UTC, is kept by atomic clocks and adjusted now and then with a leap second to stay close to the earth's rotation. Scientists, pilots and computer systems use UTC as the official world standard, while GMT survives as a time zone name and in common speech.
In practice, "GMT+2" and "UTC+2" mean the same thing on a settings screen.
GMT offsets and US time zones
Each US zone sits a set number of hours behind GMT. Daylight saving time moves each one an hour closer.
| Zone | Standard time | Daylight time |
|---|---|---|
| Eastern | GMT-5 | GMT-4 |
| Central | GMT-6 | GMT-5 |
| Mountain | GMT-7 | GMT-6 |
| Pacific | GMT-8 | GMT-7 |
Work email: "The call is at 14:00 GMT." In winter, that is 8:00 a.m. in Chicago and 6:00 a.m. in Los Angeles. In summer, the UK switches to British Summer Time, so check whether the sender meant GMT or London time.
What does GMT stand for in watches?
On a watch dial or in a product name, GMT points to a feature: a fourth hand that circles the dial once every 24 hours instead of every 12. Paired with a 24 hour scale on the bezel or the dial, it lets the wearer read a second time zone at a glance, such as home time while traveling. The style became popular with airline pilots who crossed many zones and needed home time and local time on one dial.
A GMT watch does not have to be set to Greenwich time. You can set the extra hand to any zone you want to follow.
Using GMT in a sentence
- "London is on GMT from late October to late March."
- "Set your laptop to GMT+1 while you are in Paris."
- "The launch window opens at 22:00 GMT."
- "His GMT watch shows both Denver and Tokyo time."
For more widely known letters, from SOS to RSVP, see the famous acronyms list.
Questions people ask
Is GMT the same as London time?
Only in winter. From late March to late October, the UK uses British Summer Time, which is one hour ahead of GMT.
What is the difference between GMT and UTC?
They show the same time for daily use. GMT is based on the sun at Greenwich, while UTC is kept by atomic clocks and is the official modern standard.
What does GMT-5 mean?
It means five hours behind Greenwich Mean Time. That matches Eastern Standard Time in the United States.
Why is it called mean time?
Mean means average. The sun's position at noon varies slightly through the year, so astronomers used an averaged time to keep clocks steady.
How do you say Greenwich?
Most people say "GREN itch," with a silent w. The letters GMT are said one by one.