Time is usually reckoned by using what as a starting point?

The Greenwich Meridian (Prime Meridian or Longitude Zero degrees) marks the starting point of every Time Zone of the time zone map. Every 15° longitude represents one hour's difference in time: (24 x 15 = 360, the degrees of a circle). A person can work out the time at every location on earth if you know how many degrees it is east or west of Greenwich, England.

Greenwich Mean Time or GMT is clock time at the Royal Observatory in Greenwich, London. It is the same all year round and is not affected by Summer Time (Daylight Saving Time) clock changes. When the sun is at its highest point exactly above the Prime Meridian, it is 1200 noon at Greenwich.

GMT is also a time zone, used by the United Kingdom of Great Britain and Northern Ireland (UK) when Daylight Saving Time is not in use, from October to March.

The Greenwich Meridian (Prime Meridian or Longitude Zero degrees) marks the starting point of every Time Zone of the time zone map.