Local times vary from place to place depending upon the longitude of the place, at the rate of 1 hour per 15 degrees of longitude.

In order to have some uniformity of time over an area of a country, time zones are chosen for a certain range of longitude.

For purpose of reference, and as an origin, Greenwich (UK) longitude, is chosen as the standard value of 0 degrees.

All local times are converted to GMT, by subtracting or adding the time difference of a local zone, depending on whether the place is East or West of Greenwich (UK).