When did the United States start using time zones? - Ask History

For ages, people used the sun to determine what time it was where they were. Every community set its clocks to noon based on when the sun reached its highest position in the sky; as a result, when it was noon in Washington, D.C., the local time in New York City was already minutes ahead. Before the days of long-distance travel, differences in local times weren’t a big deal. That changed with the rise of railroads in the 1800s. Although it was now possible to travel significant distances faster than ever before, a multitude of local times, particularly in large countries such as the United States, made things confusing when it came to train schedules.

On November 18, 1883, America’s railroads began using a standard time system involving four time zones, Eastern, Central, Mountain and Pacific. Within each zone, all clocks were synchronized. The railroad industry’s plan was adopted by much of the country, although the time-zone system didn’t become official across the United States until the passage of the 1918 Standard Time Act, which also established daylight saving time. By the mid-20th century, most of the world had adopted a system of international time zones, in which the planet is divided into 24 zones spaced at intervals of approximately 15 degrees of longitude. In 1884, delegates from more than two dozen nations met at the International Meridian Conference, held in Washington, D.C., where they chose the line of longitude running through the Royal Observatory in Greenwich, England, as the official prime meridian, or zero point of longitude. (Traveling east from the prime meridian, you gain an hour with each time zone; journeying west, time moves backward by an hour with each zone.)
Today, Russia, the world’s largest nation in terms of land mass, has 11 time zones, while China, the fourth-biggest by area, has just a single zone. Before 1949, the Chinese had five time zones, but after the Communist Party came to power in 1949 the government required the entire country to operate on Beijing Standard Time for the sake of national unity.