Time Zone

how time works · planet Earth · geography

Why we need them. Before time zones were invented, every city kept its own local time based on where the sun was. When trains started connecting distant cities, people got confused about what time to meet. Time zones solved this problem by letting large areas use the same time.

Based on longitude. Time zones roughly follow invisible lines called longitude that run from the North Pole to the South Pole. The world is divided into 24 zones, one for each hour of the day, because Earth rotates and shows different sunlight in different places.

Following country borders. Time zones don't always follow the longitude lines perfectly. Countries and regions arrange their borders and time zones in ways that make sense for their communication and trade, even if it means the time is a little bit off from the sun's actual position.

Changing by the hour. As you travel east or west across the world, you move from one time zone to another, and the clocks change by one hour for each zone. A person flying from New York to London crosses five time zones and arrives seven hours ahead on the clock, even though the flight only took seven hours.

Daylight saving time. Some places add an extra hour in spring and remove it in fall to make better use of daylight hours. This means the clocks in those places actually shift twice a year, which can be confusing when you're traveling or calling someone far away.