How Time Zones Work: A Simple Explanation

Why the world has time zones, how UTC and GMT work, the International Date Line, and why some places are a half hour off. A plain-English guide for students and teachers.

The sun rises in the east and moves across the sky, so it can't be noon everywhere at once. Time zones let every place keep clocks that roughly match the sun, so noon is around midday and midnight is in the middle of the night.

Why 24 time zones?

Earth turns a full 360° every 24 hours, which is 15° of longitude every hour. So in theory the world splits into 24 slices, each 15° wide and one hour apart. On the day and night map, the faint vertical lines mark those 15° slices.

In practice, borders bend around countries and states so that a whole region can share one time. That's why time zone maps look jagged rather than straight.

Before time zones: railway time

Until the 1800s, each town set its clocks by the sun, so neighbouring towns could be minutes apart. Railways made that impossible to schedule. British railways adopted Greenwich time in the 1840s, and US and Canadian railways introduced standard time zones on November 18, 1883. Countries agreed to use Greenwich, England as the world's starting point at a conference in 1884.

UTC and GMT

Every time zone is described as an offset from UTC (Coordinated Universal Time), the world's reference time kept by atomic clocks. New York is UTC−5 in winter; Tokyo is UTC+9. GMT (Greenwich Mean Time) shows the same time as UTC and is the name of the UK's winter time zone. Pilots and the military call UTC Zulu time.

The International Date Line

Go east 12 time zones from Greenwich and west 12 time zones, and you meet on the other side of the world, a full day apart. The International Date Line, roughly along 180° longitude in the Pacific, is where the date changes. Cross it going west and you skip forward a day; going east, you repeat a day. Kiribati moved the line in 1995 so its islands would share one date, which is why it now has places at UTC+14, the earliest time on Earth.

Odd time zones

• India uses UTC+5:30, a half hour between its neighbours, so the whole country can share one time. See IST.
• Nepal is UTC+5:45, one of the few zones offset by 45 minutes. See NPT.
• China is about as wide as the contiguous US but uses a single time zone, so in the far west the sun can rise after 9 AM.
• Newfoundland in Canada is UTC−3:30.

Daylight saving time

Many countries move their clocks forward an hour in summer to get more daylight in the evening, then back in autumn. Most of the world doesn't do it. See this year's dates.

Try it

• Check your own time zone and its offset.
• See the world's strangest time zones and how the date line works.
• Read the full story of railway time and 1884.
• Compare cities on the world clock.
• See how many time zones each country has.

For teachers

Every tool on this site is free with no sign-up, and works on classroom screens and Chromebooks. The day and night map shows the terminator moving in real time, the classroom timer is readable from the back of the room, and you can embed a live clock on a class website.