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 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 zone which sets the time of day and is at the centre 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 Although GMT has been replaced by atomic time (UTC) it is still widely regarded as the correct time for every international time zone.

Taken From: http://wwp.greenwichmeantime.co.uk/what-is-gmt.htm

I heard from my dad that the GMT time zone is measured by the distance/latitude you are away from Greenwich. The earth is divided into 24 timezones. Each time zone is one hour, as such, each zone is 360 deg/24 = 15 deg. Therefore, if Singapore is 8 hours ahead of Greenwich, we are at least (15x7)105 deg from Greenwich. However, he said Singapore is actually 7.5 hours ahead and therefore we should be at least (105-7.5) 97.5 deg from Greenwich. I did a quick check and we are actually 103 deg 50 min from GMT.

Why do we have different timezones?
Because, the earth spins on its own axis. Therefore, a countries day could be another's night.

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 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 zone which sets the time of day and is at the centre 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 Although GMT has been replaced by atomic time (UTC) it is still widely regarded as the correct time for every international time zone.

Taken From: http://wwp.greenwichmeantime.co.uk/what-is-gmt.htm

I heard from my dad that the GMT time zone is measured by the distance/latitude you are away from Greenwich. The earth is divided into 24 timezones. Each time zone is one hour, as such, each zone is 360 deg/24 = 15 deg. Therefore, if Singapore is 8 hours ahead of Greenwich, we are at least (15x7)105 deg from Greenwich. However, he said Singapore is actually 7.5 hours ahead and therefore we should be at least (105-7.5) 97.5 deg from Greenwich. I did a quick check and we are actually 103 deg 50 min from GMT.

Why do we have different timezones?
Because, the earth spins on its own axis. Therefore, a countries day could be another's night.