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U.S. Time Zones
The four clocks of the contiguous country
The contiguous United States is divided into four standard time zones - Eastern, Central, Mountain, and Pacific - each roughly 15 degrees of longitude wide and an hour apart, with Alaska and Hawaii keeping two more zones farther west. The system was created by the railroads in 1883 to replace a chaos of local "sun times" that had made scheduling almost impossible, and it was written into federal law in 1918.
The zone boundaries zigzag rather than following straight meridians, bending around state lines and communities so that a city and its economic region stay on a single clock. A few places opt out of daylight saving time, most famously most of Arizona. The marker here sits near where the Central and Mountain zones meet on the Great Plains - one of the lines where travelers reset their watches by an hour as they cross the country.