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GMT is short for Greenwich Mean Time, an international reference point for time zones.

How is GMT pronounced?

[ gee-em-tee ]

Greenwich Mean Time (GMT) is named after Greenwich, a borough outside London, where the British Royal Observatory has kept time since 1675 to aid sailors traveling to the Americas.

Mean time refers to mean solar time, an old method of calculating time based on the position of the sun. Greenwich marks the average time it takes earth to rotate around the sun from noon to noon.

The phrase Greenwich Mean Time is recorded in 1782, the abbreviation GMT in 1840.

In the 19th century, the United Kingdom sought a national time system to use for its railways in place of the numerous local times used by towns and cities, designating GMT in 1880. The United States adopted it in 1883. In 1884, British mapmakers established Greenwich as the prime meridian, where longitude is measured as being 0 degrees.

That same year, 24 time zones were created around the world with GMT as its reference point. Other time zones are plus or minus (ahead of or behind) GMT, making, for instance New York City GMT-5 (hours) but Paris GMT+1.

In 1960, a new universal time standard, now called Coordinated Universal Time (UTC), was mapped onto GMT. They share the same time practice, just that GMT is used as a national time zone by the UK, Ireland, and some African countries, while UTC is simply a reference.

The International Space Station still uses GMT, and GMT is still considered an accurate time in all time zones—pretty impressive considering it is over 300 years old.

Space is Big, Space is Public – watch coverage of the Public Day, live from Bremen, including a call with ESA astronaut on the . Starts 11:30 GMT (13:30 CEST). (public day session is free of charge)

— ESA (@esa)

GMT is the time zone that the U.K. and Ireland, among some other countries, uses outside of Daylight Saving Time (DST). Time just has to be complicated, doesn’t it?

At 93, Mahathir Mohamad is the world's oldest elected leader.

Can he salvage Malaysia and lead it back to a brighter future?

He speaks to on BBC World News today at these times

14:30 GMT
19:30 GMT

— BBC News (World) (@BBCWorld)

Despite the existence of nation-independent UTC, it is still common practice for people to refer to their own time zone in reference to GMT. GMT is very often used to indicate the time of live events (such as a sports broadcast) for international audiences.

Slide into patch v6.01 on Wednesday, October 3! Patch Notes will be available when downtime begins at 3 AM ET (0700 GMT).

— Fortnite (@FortniteGame)

✈️I 3h30 de vol
🕐I Heure locale 2️⃣0️⃣h (GMT+3)
🌤I 23°

— Olympique de Marseille (@OM_Officiel)

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On the occasion of the celebration of , join our interview with and learn more about the culture of peace, yoga & what traditions can do to tackle the 🌍's problems

Make sure to tune in on 3 October at 10am GMT+2

▶️

— UNESCO (@UNESCO)

More examples of GMT:

“The Wallabies have lost six of their last seven tests. A loss to the Springboks would drop them to their lowest ranking in 15 years. The Rugby Championship match starts at 1005 GMT.”
—Associated Press, September 2018

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This content is not meant to be a formal definition of this term. Rather, it is an informal summary that seeks to provide supplemental information and context important to know or keep in mind about the term’s history, meaning, and usage.