Greenwich Mean Time or Meridian Time has been a time standard since 1884. It was chosen because the Prime Meridian (the line of 0° Longitude) runs through Greenwich.
Greenwich was a royal park and palace on a hill to the south of the River Thames east of London.
In 1675 the great race to create accurate maps for navigators had begun and Charles II offered the land to The Royal Society for Britain's first national observatory. Christopher Wren was commissioned to design the domed building and John Flamsteed was appointed the Astronomer Royal. British mapmakers began to set Longitude from Greenwich and in 1884 it was adopted as the international meridian or starting point.
Every 15° longitude represents one hour's difference in time: (24 x 15 = 360, the degrees of a circle). You can work out the time at every location on earth if you know how many degrees it is east or west of Greenwich.
The Greenwich Meridian (Prime Meridian or Longitude Zero degrees) marks the starting point of every time zone in the World. GMT is Greenwich Mean (or Meridian) Time is the mean (average) time that the earth takes to rotate from noon-to-noon.
GMT is World Time and the basis of every world Time Zones of the time zone map. GMT sets current time or official time around the globe. Most time changes are measured by GMT. Although GMT has been replaced by atomic time (UTC) it is still widely regarded as the correct time for every international time zone.