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 Although GMT has been replaced by atomic time (UTC) it is still widely regarded as the correct time for every international time zone.
Back to the time converter, one thing required is that you need to know your own GMT offset. For example, I'm on the west coast of the U.S., on Pacific Standard Time (PST), and our offset is -8. When we are adjusted for daylight savings (PDT), we are offset by -7. All locations west of Greenwich have a negative offset, and all east of Greenwich have a positive offset. The meeting point between east and west is the International Date Line.
The International Date Line (IDL) is an imaginary line on the surface of the Earth opposite the Prime Meridian which offsets the date as one travels east or west across it. Roughly along 180° longitude, with diversions to pass around some territories and island groups, it mostly corresponds to the time zone boundary separating +12 and −12 hours GMT (UT1). Crossing the IDL traveling east results in a day or 24 hours being subtracted, and crossing west results in a day being added.How do you keep all these time zones and offsets straight? Well, return the the Greenwich Mean Time site and using the left-hand navigation drill-down to your region of the world.