Imagine getting on a train and arriving at a station where the clocks showed a completely different time. That is exactly what happened in America before 1918. Every town set its own clocks based on when the sun was directly overhead. A town fifty miles to the west might be four minutes behind. This created chaos for railroads. Train schedules were nearly impossible to follow. On March 19, 1918, Congress passed the Standard Time Act. This law divided the country into official time zones. The four main zones are Eastern, Central, Mountain, and Pacific. Each zone covers a wide strip of land running from north to south. The idea was not completely new. Railroads had already created their own time zones in 1883, but those were unofficial. The 1918 law made them part of federal law. The act also introduced daylight saving time to the United States for the first time. The time zone system works because Earth rotates. As the planet turns, different parts face the sun at different times. The zones are based on longitude, the imaginary lines running from the North Pole to the South Pole. The United States spans about four hours of time difference.