Standard time is the official clock time used across a specific region or time zone, set by agreement rather than by the sun's position in the sky. Before standard time was invented in the 1880s, every city set its clocks by when the sun reached its highest point, which meant that noon in Boston was several minutes different from noon in New York. This worked fine when people traveled slowly by horse and wagon, but when railroads started connecting cities, the chaos became impossible to manage. A train schedule that said “arrives at 3:15” meant something different in every town along the route.

Standard time solved this problem by dividing the world into time zones, with everyone in the same zone setting their clocks to exactly the same time. The United States has four main time zones: Eastern, Central, Mountain, and Pacific. When it's 3:00 PM Eastern Standard Time in New York, it's 2:00 PM Central Standard Time in Chicago, 1:00 PM Mountain Standard Time in Denver, and 12:00 PM Pacific Standard Time in Los Angeles.

Many places also use daylight saving time during summer months, shifting clocks forward one hour to have more daylight in the evening. When daylight saving time ends in fall, we return to standard time.