Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory, Greenwich where a system was first developed around 1850 for tracking time based on the rotation of the Earth. It is now often used to refer to Coordinated Universal Time (UTC) when this is viewed as a time zone.

Strictly speaking, UTC is not a time zone but an atomic time scale which only approximates GMT in the old sense. It is also used to refer to Universal Time (UT), which is an astronomical concept that directly replaced the original GMT.

In 1970 the Coordinated Universal Time system was devised by an international advisory group of technical experts within the International Telecommunication Union (ITU). UTC is the International Atomic Time (TAI, from the French Temps atomique international) with leap seconds added at irregular intervals to compensate for the Earth's slowing rotation. Leap seconds are used to allow UTC to closely track UT1, which is the mean solar time at the Royal Observatory, Greenwich.

The ITU felt it was best to designate a single abbreviation for use in all languages in order to minimize confusion. Since unanimous agreement could not be achieved on using either the English word order, CUT (coordinated universal time), or the French word order, TUC (temps universel coordonné), the acronym UTC was chosen as a compromise.

The difference between UTC and UT1 cannot exceed 0.9 s, so if high precision is not required, the general term Universal Time (without a suffix) may be used.

In casual use, Greenwich Mean Time (GMT) is the same as UTC and UT1. Owing to the ambiguity of whether UTC or UT1 is meant, and because timekeeping laws usually refer to UTC, GMT is avoided in careful writing.

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Stands for Greenwich Mean Time Coordinated Universal Time (Abbreviated From The French Name)
Refers To A Time Zone A System Of Time-Keeping
Usage By Human Readable Clocks By Digitally Synchronized Clocks
Measured Using Rotation Of Earth (Historically) Atomic Transition Principle (Periodic updates with astronomical time)

edit Time in Greenwich, UK

In the community of Greenwich, GMT (in the form of UTC) is the official time only during winter (during summer the time in Greenwich is British Summer Time rather than "GMT").

The Shepherd Gate clock at the Royal Observatory in Greenwich.
The Shepherd Gate clock at the Royal Observatory in Greenwich.

edit Time Zones Around the World

The video below provides a brief history of time zones in the world, including the use and development of GMT and UTC. It also discusses instances of "strange" time zones, such as time zones presenting half-hour differences, and countries or regions refusing to adopt traditional time zones.

edit References

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Comments: GMT vs UTC

Anonymous comments (5)

January 17, 2014, 8:04am

Some of the Facebook replies appear to be the direct result of heavy intoxication. UTC is not a "time zone" at all - as is GMT. It is simply a method of time keeping. Also, if you happen to have been born after the year 1990 chances are you are completely incapable of accurately reading a standard analog clock anyway, so you need not worry yourself with any of this time business - just check your phone and know someone smarter than you has paved the way for your ignorance.

— 74.✗.✗.230

January 17, 2013, 1:33am

actually the maximum permissible deviation between GMT amd UTC is 0.9 sec (i.e., nine tenths of one second) rather than 9 sec.(nine seconds) . Were it anything approximating a maximum of nine seconds, much confusion would ensue.

— 207.✗.✗.76

August 29, 2013, 8:05pm

In reference to the comment about GMT and UTC differing in the summer for 7 months, this is inaccurate. While the time in Greenwich may change to British Summer Time (BST) during the summer, actual GMT does not change with Daylight Savings Time. The UK changes back to GMT in the winter.

— 209.✗.✗.25

February 1, 2013, 9:39pm

First commenter is confusing GMT with British Summer Time, I think.

— 72.✗.✗.66

April 26, 2012, 2:48am

I Have just learnt after years of confusion that the main difference between the two terms
( apart From the 9 sec. max dev. ) relates to the fact that UTC cannot change in sync with GMT at the changeover point for GMT to enter Daylight Saving Time therefore a one hour difference appears at that point in time for a period of 7 months.

— 14.✗.✗.51


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