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Greenwich Mean Time (GMT)

Time zone

Greenwich Mean Time, GMT

UTC offset

Greenwich Mean Time (GMT) is in the same time zone as Coordinated Universal Time (UTC).
Important note:
Most cities located in Greenwich Mean Time (GMT) zone currently observe Daylight Saving Time (DST).
Therefore, most cities there are using British Summer Time (BST), Irish Standard Time (IST).

Example of location: London
Use the search bar above to look up by city and not by time zone.

History and Overview

With so many different types of time zones and variations on standard time, it’s hard to keep track of them all. One of the best place to start with is Greenwich Mean Time. This standardized time zone doesn’t have any offset from Universal Coordinated Time (UTC) like you would normally expect. Instead, it’s a zone used in Europe, Africa, North America, and Antarctica.

To better understand this concept, we’ll explore the meaning behind it, then we’ll compare GMT to UTC to help you understand the distinction between the two. Once that’s finished, we’ll go into detail about the history of GMT and how it has molded the way we tell time around the world today.

A Broad Description of GMT

Humans have pursued an accurate way of telling time since the dawn of our history. We have always looked to the stars for answers to these questions. The solstices and equinoxes provided points of reference for us to use in our pursuit. For over 1,300 years the sun’s position in a local area was the basis for the time of day. The sun was always due east at 6 AM, above your head at noon, and in the west at 6 PM.

The Royal Greenwich Observatory started the process for a unified standard time by becoming the Prime Meridian or the zero longitude for the world. It wasn’t until later in history though, when travel and railways created the need for a universal standard.

The abbreviation GMT refers to the mean solar time at the Royal Observatory in Greenwich, London. Solar time itself is a simple concept to grasp: it’s a method of telling time based on the Sun’s position in our sky. There is apparent solar time, which is based on things like sundials, then there’s mean solar time, which is based on clocks.

GMT is known as the global time standard. Universal Coordinated Time (UTC) is structured so that it’s no more than 0.9 seconds faster or slower than GMT. The name is used by the United Kingdom and its major bodies such as the BBC World Service, the Royal Navy, the Met Office and others in Arab countries such as the Middle East Broadcasting Center and OSN.

GMT is a term also used by the countries of the commonwealth which include the following locations:

  • Australia
  • New Zealand
  • South Africa
  • India
  • Pakistan
  • Malaysia
  • Ireland
  • Greenland

There are of course other countries in the Eastern Hemisphere that also use the term. An equivalent title for GMT is Universal Time (UT) which is a concept used by many technical fields. In the summer, British Summer Time is used. During these months, the time is exactly one hour ahead of GMT. During the winter months, GMT is the legal time in Britain.

Despite being solar time, noon is rarely the moment when the sun crosses the Greenwich meridian. Earth has an uneven speed in both its ecliptic orbit and its axial tilt. In some cases, the sun’s highest point may be up to sixteen minutes away from noon measured in GMT. This time is the annual average for this event which is why the “M” stands for “mean” in the title.

In the past, GMT has been a term with two different meanings. On the one hand it’s used to number the hours from midnight or noon. The more specific and conventional meaning is to use midnight as the zero hour of each and count upwards from there. There are other time zones within UTC +0:00 as well. These are listed below:

  1. AZOST - Azores Summer Time

This time zone has no offset from UTC and is used by the Atlantic island of Azores near Portugal. During the winter months AZOT is observed which is UTC -1:00.

  1. EGST - Eastern Greenland Summer Time

This time zone with a zero offset from UTC is observed by Greenland. During the winter months, EGT is observed which is UTC -1:00. This switch is only observed by a very small part of Greenland’s east coast.

  1. WET - Western European Time

This time zone has no offset from UTC and is observed by several European countries during the winter. The WEST time zone is used in the summer. The places where this time zone is observed are the Canary Islands, the Faroe Islands, and Portugal. In Africa, Morocco and the Western Sahara use this zone.

  1. Z - Zulu Time Zone (Military)

This military zone has a zero offset from UTC and is used for military purposes in aviation. It’s location is over the sea between the longitudes of 7.5 degrees west and 7.5 degrees east.


The Differences Between GMT and UTC

While many people will interchange GMT and UTC as if they were the same thing, the fact remains that they are not. GMT is a time zone and a reference point, but UTC is a standard that has no set zone. The time difference between these two concepts is minimal, but the differences still remain.

  • GMT is a time zone that is used officially in many European and African countries. It is also used on the International Space Station. It is displayed either in a 12 hour or 24 hour format (i.e 0-24 or 1-12 AM/PM).
  • UTC on the other hand is not a time zone at al. Instead it is a time standard that is used as the basis for civil time around the world. No country or state uses UTC as their local time. Everything is an offset of UTC to some extent.

In regards to Daylight Saving Time, neither UTC nor GMT changes for these summer months. Instead, countries that participate in the practice move to a new time zone temporarily during the time period.

The History of Greenwich Mean Time (GMT)

In its early history, the United Kingdom evolved into a maritime nation, which is any nation that borders the sea and is dependent on it for a majority of the following activities:

  • Commerce
  • Transport
  • War

This evolution resulted in British mariners using chronometers set to GMT which they could use to find their longitude from the Greenwich meridian which is the basis for zero degrees longitude. These chronometers were portable time standards that allowed the mariners to utilize celestial navigation to find their longitude and therefore their location. The Prime Meridian’s location was decided at the International Meridian Conference of 1884. GMT was officially adopted by Great Britain in 1847 when the Railway Clearing House started using it.

Later that following year, the term “railway time” was created. In 1880 GMT was adopted as the main time standard for all of Great Britain’s purposes. The Isle of Man adopted the standard in 1883, Jersey followed in 1898, and Guernsey in 1913. This was followed by Ireland in 1916. Prior to the use of electronic time signals, the Greenwich Observatory used something called a time ball.

This time ball was a large painted sphere that was dropped at predetermined times to give ship navigators an opportunity to calibrate their chronometers at sea. A modern day example of such a time ball is the iconic Times Square Ball that has been used every New Year’s Eve since 1907 in New York City to signal the onset of the New Year. Today there are still sixty time balls to be found around the world, though most of them have been decommissioned.   

The Earth’s rotation isn’t constant in speed. In fact, it is slowly down slightly, which as made solar mean time less reliable. This issue was fixed with the invention of the atomic clock. These devices measure the energy output of electronic in an environment close to absolute zero temperatures to measure time more accurately than any other device in existence. 

In 1972, GMT was replaced with Coordinated Universal Time which was defined by a number of atomic clocks synchronized around the world. Since this change, there has been a slow movement away from the traditional standards set by GMT. Howes in a 1997 book described this change:

“Indeed, even the Greenwich meridian itself is not quite what it used to be—defined by "the center of the transit instrument at the Observatory at Greenwich". Although that instrument still survives in working order, it is no longer in use and now the meridian of origin of the world's longitude and time is not strictly defined in material form but from a statistical solution resulting from observations of all time-determination stations which the BIPM takes into account when coordinating the world's time signals.

Nevertheless, the line in the old observatory's courtyard today differs no more than a few meters from that imaginary line which is now the prime meridian of the world.”

Final Thoughts

Greenwich Mean Time played a major role in helping develop our most accurate methods of keeping time and of course, in developing time zones. It lives on as a time zone in the Eastern Hemisphere, but it will always be the stepping stone that took our world’s time standard from good to great.