Time is measured in different places around the world
Greenwich is an observatory in London England, assigned as the "starting point" for the time zones.
Strictly speaking, time can be measured in any city, but you are probably thinking of the international time zones, for which the city of Greenwich, England is the official starting point.
All time zones are set by Coordinated Universal Time (UTC).
It is a map showing the time zones in different parts of the world, usually when it is midday in Greenwich, from where the time zones are measured from. Most of the map is done in sections of 15°, representing one hour, although there are half hour time zones in some parts of the world.
The city of Greenwich - near London. ALL timezones across the globe are measured according to the time in Greenwich, England. However - GMT is now called UTC - Universal Time Constant.
All continents have time zones. It is just that the continents are so big that they have many time zones in them.
There are 24 time-zones around the planet. Measured from Greenwich (near London) in England - they ensure the whole planet lives, sleeps and works relative to the hours of daylight we have. Greenwich Mean Time (now called Universal Time Constant or UTC) is the time zone for the UK, and the standard by which all other time zones around the planet are relative to. There are four time zones in america - Pacific, Central, Mountain & Eastern
Check out the related interactive link to find time zones around the world. http://www.worldtimezone.com/
The meridian of time is an imaginary line that represents the prime meridian (0° longitude) from which time zones are measured. It is used as a reference point for coordinating time globally.
G.M.T. is Greenwich Mean Time.Greenwich, England has been the home of Greenwich Mean Time (GMT) since 1884. GMT is sometimes called Greenwich Meridian Time because it is measured from the Greenwich Meridian Line at the Royal Observatory in Greenwich. Greenwich is the place from where all time zones are measured.
G.M.T. is Greenwich Mean Time.Greenwich, England has been the home of Greenwich Mean Time (GMT) since 1884. GMT is sometimes called Greenwich Meridian Time because it is measured from the Greenwich Meridian Line at the Royal Observatory in Greenwich. Greenwich is the place from where all time zones are measured.
The prime meridian is the line that passes through Greenwich, England - it 'splits' the Earth into the Eastern and Western hemispheres - and is the line by which all time zones around the planet are measured from.