Longitude and Time
rom the diagram it can be seen that if it is noon along the prime or Greenwich meridian of longitude, it will be 1:00 pm 15° to the east and 11:00 am 15° to the west. These times are referred to respectively as being "ahead" or "behind" Greenwich Mean Time (GMT), or time along the prime meridian. Using this principle, it is easy to determine longitude. For example, if a chronometer set on Greenwich Mean Time reads 6:00 pm or 1800 GMT at the time when the sun reaches its highest point in the sky (local noon) from the vantage point of the observer, then the sun reached its zenith over the prime meridian 6 hours earlier. The observer is, therefore, 90° west of prime meridian (6 hours x 15° = 90° of longitude). Since local sun time is different everywhere from GMT, trying to keep track of train schedules on east-west routes became impossible by the mid-19th century and so, by international convention in 1877, standard time zones of approximately 15° of longitudinal width were established, centered on 0°, 15°E and W, 30°E and W, and so on. Within each time zone, time is the same everywhere, rather than being kept locally.
--John Logan Allen