Difference Between GMT and UTC

GMT vs UTC | Greenwich Mean Time and Coordinated Universal Time | Atomic Time


GMT is Greenwich Mean Time whereas UTC is Coordinated Universal Time. GMT refers to the time maintained at the Royal Observatory in Greenwich, London called the mean solar time. UTC is based on International Atomic Time (TAI).

Greenwich Mean Time is is based on astronomical observations, it is used primarily by bodies associated with the United Kingdom like the BBC World Service, the Royal Navy and the Met Office. On the other hand, Coordinated Universal Time (UTC) is an international time scale recommended by the International Bureau of Weights and Measures (BIPM) as the legal basis for time. It is a method of measuring time using atomic clocks. To determine the international standard of Coordinated Universal Time, the Bureau of Weights and Measures (BIPM) in Paris Coordinates data from atomic clocks located in timing laboratories around the globe. It is interesting to note that leap seconds are added to UTC to compensate for the slowing rotation of the Earth.

It is important to know that leap seconds are used to allow UTC to track the mean solar time in the Royal Observatory in Greenwich, London.

As a matter of fact several countries adopt GMT in their legislation. They are the United Kingdom, Belgium, Republic of Ireland and Canada. UTC on the other hand is the time standard used for several Internet and World Wide Web standards. It is also the basis of the satellite global positioning system (GPS). UTC is used by the Network Time Protocol, created to synchronize the clocks of many computers over the Internet. The difference between Greenwich Mean Time and Coordinated Universal Time is measured in fractions of a second. The difference is important, however, in scientific matters.

Thus it can be said that the UTC is Internet based time standard whereas GMT is country based time standard. In casual use when fractions of a second are considered not very important, GMT can be taken equivalent to UTC. It is often thought that time zones around the world are positive or negative offsets from UTC. In fact is quite true that UTC replaced GMT as the main reference time scale in various regions.

The International Telecommunication Union otherwise called ITU thought that it was best to have a single abbreviation for use in all the languages so that confusion can be curbed to a great extent. In fact they could not arrive at a conclusion whether to have the English word order or the French word order and hence as a result the acronym UTC was chosen.

Images Courtesy: 

  1. Time by Alan Cleaver (CC BY 2.0)