Throughout most of written history, time keeping was essentially a local affair. Every town set its clocks according to the motions of the Sun, and Noon was defined as the time when the Sun reached its maximum altitude above the horizon. Each town had its own official timekeeper to calibrate a town clock to the Sun's motions. This town clock represented the "official time" in that town, and the citizens would set their own watches and clocks accordingly.

This system of local timekeeping was sufficient so long as the majority of people stayed in one place most of their lives. But, as technology allowed people to move across the country, a new system became necessary. In the United States and Canada especially, the railroads found it near impossible to construct accurate timetables for stops, since each stop was on a different time. Timetables could only become more efficient if every city and town along the rail network adopted some standard method of keeping time.

In 1878, Canadian Sir Sanford Fleming proposed that the globe be divided into 24 time zones, each 15 degrees of longitude in width. Since the Earth rotates once every 24 hours on its axis and there are 360 degrees of longitude, each hour of Earth rotation represents 15 degrees of longitude. American railroad companies began using Fleming's time zones in 1883. In 1884, members of the International Prime Meridian Conference agreed that the longitude of Greenwich, England, would become zero degrees longitude and established the 24 time zones relative to the "Prime Meridian."

Today, most nations use the time zones suggested by Fleming. The map below shows all 24 of the time zones currently in use. National boundaries and political matters influence the shape of the time zone boundaries. China, for example, uses a single time zone instead of the five it "should" have. The second map shows the six different time zones in use within the borders of the United States.

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This page was last updated on 08/17/2018.