GMT stands for Greenwich Mean Time or Greenwich Meridian Time. GMT is the starting point for all time zones. Established in 1884, it is measured from the Prime Meridian Line (zero degrees longitude) at the Royal Observatory in Greenwich, England. The globe is divided into 24 designated but invisible timezones with each zone offset by one hour of time. These timezones are periodically adjusted on the map, generally to the West of the original line.
GMT is established as timezone zero (0), with twelve zones on either side of the Prime Meridian. Each zone to the West of GMT is identified by a negative (-) identifier, (-1, -2, -3...). Zones to the East of GMT have a positive (+) sequential identifier, (+1, +2, +3...). When figuring the difference in timezones, add one hour for each zone West-to-East and subtract one hour for each zone East-to-West.
For example, if you look at a timezone map, California, USA is GMT-8 and New York, USA is GMT-5. Add one hour for each of the three timezones between the two locations if you are located in California, or subtract one hour for each, if you are located in New York.
Calculating timezone differences internationally is figured the same way, even if the destination is on the opposite side of GMT 0, where it changes from a negative to a positive integer. If you are located in GMT -8 and you are trying to determine what time it is at a location in China (GMT +8), you would count 16 timezones difference. As you are going from West-to-East the clock moves forward 16 hours. If it is reversed and you are in China, calculating East-to-West, the clock would move back 16 hours.
To determine if the destination time is AM or PM, you will need to take note of when the clock passes 12:00 and adjust it accordingly. The calendar date may need to be adjusted even if the International Date Line is not crossed.
If the destination is to the East of the Date Line and you are on the opposite side, it will be one calendar day behind your local date. If the destination is to the West of the Date Line from your location, they will be one calendar day ahead of you. Other than the date difference, the clock time is calculated as normal.
