A time zone is a region of Earth where everyone uses the same clock time. The world is divided into 24 time zones, one for each hour of the day, because the sun can't be directly overhead everywhere at once.

Here's why time zones exist: when it's noon in New York (with the sun high in the sky), it's already midnight in Tokyo (where the sun set hours ago). Without time zones, we'd all use the same clock time, which would mean some people would call the middle of the night “noon” while others experienced actual midday sunshine at “noon.” That would be hopelessly confusing.

Time zones follow roughly vertical lines running from the North Pole to the South Pole, though they bend and curve to match country borders. When you travel east across a time zone boundary, you set your clock forward one hour. Travel west, and you set it back. This is why flying from California to New York means “losing” three hours: if you take off at 9 AM Pacific time and fly for five hours, you land at 5 PM Eastern time, not 2 PM.

The system lets everyone experience roughly the same relationship between clock time and sunlight, no matter where they live. When it says 3 PM on your clock, the afternoon is well underway whether you're in London, Lagos, or Los Angeles.