Greenwich Mean Time or GMT is the time standard against which all other time zones in the world are referenced.
It is the same all year round and is not effected by Summer Time or Daylight saving Time.
GMT was originally set-up to aid naval navigation when the globe started to open up with the discovery of the "New World" (America) in the fifteenth Century.
It was not forced on to "land-lubbers" until the introduction of the railways (railroads) in the mid nineteenth century. In the 1840 's with the introduction of the railways (railroads) there was a need, in Britain, for a national time system to replace the local time adopted by major towns and cities. Greenwich was the national centre for time and had been since 1675 . It was not adopted officially by Parliament until 2 August 1880 .
Greenwich Mean Time (GMT) was adopted by the United States (USA) at noon on 18 November 1883 when the telegraph lines transmitted time signals to all major cities. Prior to that there were over 300 local times in the USA.
Greenwich Mean Time (GMT) was adopted universally on the 1 November 1884 when the International Meridian Conference in Washington, DC, USA met. From then the International Date Line was drawn up and 24 time zones created.
Today, GMT is used as the UK's civil time, or UTC. But to navigators, GMT has referred to "UT1", which directly corresponds to the rotation of the Earth, and is subject to that rotation's slight irregularities. It is the difference between UT1 and UTC that is kept below 0.9s by the application of leap seconds.