Using Greenwich Mean Time (GMT)

Time was based on how many time zones separated a location from the Royal Greenwich Observatory in the UK. Greenwich Mean Time was determined by the average time of day when the sun passed over the Prime Meridian at Greenwich.

On Nov. 18, 1883, railroads in North America converted to a system of just over four time zones: eastern time, central time, mountain time, and Pacific time. Eventually, it became the standard across the U.S.