Time zones are slices of the Earth, each roughly 15 degrees of longitude wide, where everyone sets their clocks to the same time, so that noon happens near the moment the sun is highest in the sky. Because the Earth spins once every 24 hours, there are 24 basic zones, and moving one zone east usually puts your clock one hour ahead.
Why is it night in Tokyo when it's lunchtime in London?
Think of the sun as a torch shining on a ball. At any moment, half the planet is in daylight and half is in darkness, and because the Earth keeps turning, places slide from the dark side into the light (sunrise) and back out again (sunset).
The Earth turns from west to east. Tokyo is far to the east of London, so it reaches morning, noon and evening many hours earlier. When people in London sit down to lunch at 12:00 in winter, the clocks in Tokyo already say 21:00, nine o'clock at night, and Japanese children are getting ready for bed.
Here is the maths that makes time zones work. A full circle has 360 degrees, and the Earth turns all the way round in 24 hours. Divide 360 by 24 and you get 15. So the Earth turns 15 degrees every hour, or one degree every 4 minutes. Geographers measure east and west with lines of longitude, which run from the North Pole to the South Pole. If you have not met them yet, our guide to latitude and longitude for kids explains how they work. Tokyo sits at about 140 degrees east and London at about 0 degrees, and 140 divided by 15 is a little over 9. That is where the nine-hour difference comes from.
Who decided that Greenwich would be the starting line?
Every system of counting needs a zero. For longitude, the zero line is called the prime meridian, and it runs through the Royal Observatory in Greenwich, a part of south-east London.
It could have been somewhere else. In the 1800s, many countries drew their maps from their own capital: France used Paris, for example. So in October 1884, delegates from 25 nations met in Washington, D.C., for the International Meridian Conference. They voted 22 to 1 to make Greenwich the prime meridian for the whole world. San Domingo (today the Dominican Republic) voted against, and France and Brazil did not vote either way.
Why Greenwich? By then, most of the world's ships already used sea charts based on it, because Britain had the largest navy and merchant fleet and its observatory published excellent star tables.
The time at Greenwich became known as Greenwich Mean Time, or GMT. Today scientists use a nearly identical standard called Coordinated Universal Time, or UTC, which is kept by atomic clocks. When you see "UTC+9" next to Tokyo, it means Tokyo's clocks are 9 hours ahead of Greenwich.
What did people do before time zones were invented?
For most of history, each town used local solar time. When the sun was highest over the church tower, the town clock was set to noon. That meant that every town a little to the east or west had a slightly different time. Bristol, in the west of England, was about 10 minutes behind London.
Trains changed that. A steam train could cross many towns in an afternoon, and a timetable that used a different time at each station was a recipe for missed connections and even crashes on single-track lines. British railways began running on London time in the 1840s.
North America had a bigger problem, because it is so wide. American railways were using dozens of different local times. On 18 November 1883, the railways of the United States and Canada switched to a system of standard time zones. People called it "the day of two noons", because many town clocks struck twelve, were wound back, and struck twelve again. The United States government did not make these zones official law until 1918.
One of the people pushing hardest for a worldwide system was Sandford Fleming, a Scottish-born engineer who built railways in Canada. A well-known story says that in 1876 he missed a train in Ireland because the printed timetable was confusing. By 1879 he was proposing 24 time zones covering the whole globe, the idea that the world eventually adopted.
Why aren't time zones straight like longitude lines?
If you look at a time zone map, the zones start out as neat stripes over the open ocean, then wiggle and zigzag as soon as they reach land. That is because each country gets to choose its own time, and countries prefer borders that make sense for people rather than for maths.
A straight line through the middle of a city would make children on one side of the street start school an hour before their friends across the road. So zone borders usually follow the edges of countries, states or provinces instead.
Some countries do not even use whole hours. India sits between two of the standard 15-degree stripes, so in 1947, after independence, it settled on one time for the whole country set halfway between them: UTC+5:30. The line India uses runs at 82.5 degrees east, close to the city of Mirzapur. Its neighbour Nepal went one step further and uses UTC+5:45, so when it is 12:00 in Delhi it is 12:15 in Kathmandu. Iran uses UTC+3:30, and parts of Australia use UTC+9:30.
Add all the half-hour and quarter-hour zones to the 24 whole-hour ones, and there are around 38 different local times in use around the world, depending on the time of year.
Which is stranger: China with one clock or Russia with eleven?
Russia is the largest country on Earth, stretching from Kaliningrad, near Poland, to the Kamchatka Peninsula facing Alaska. It uses 11 time zones, from UTC+2 in Kaliningrad to UTC+12 in Kamchatka. Russia tried cutting the number down to 9 in 2010, but people in some regions complained about dark mornings, and it went back to 11 in 2014.
China is about 5,000 kilometres wide from east to west, enough to cover roughly five of the 15-degree slices. Yet since 1949 the whole country has used one time, Beijing Time, UTC+8. In Kashgar, in the far west, the winter sun may not rise until about 10 a.m. by the clock. Many people there unofficially live by "Xinjiang time", two hours behind Beijing. Our article on China's single time zone has more about how daily life works there.
So which is stranger? Russia's clocks follow the sun closely, but phoning across the country in office hours is hard. China's single clock is simple for timetables, but in the west its clocks and its sunshine disagree by two hours or more, which makes China the odder case.
Neither country holds the record, though. That belongs to France. Mainland France has one time zone, but with territories from French Polynesia to Réunion it spans 12. You can see the full ranking in our list of countries with the most time zones.
Where does tomorrow start?
If you keep travelling east, adding an hour every time you cross a zone, you will eventually have added a whole day. Somewhere, the calendar has to jump. That place is the International Date Line, which runs roughly along the 180-degree line of longitude in the middle of the Pacific Ocean, on the opposite side of the world from Greenwich.
Like time zones, the date line bends to avoid splitting countries. It zigzags around Russia's eastern tip and around groups of Pacific islands. Two island nations have moved it on purpose:
- Kiribati is made up of 33 islands spread over a huge stretch of ocean, and the date line used to run right through it. Its western islands were a whole day ahead of its eastern ones, so government offices could only talk to each other on four weekdays. On 1 January 1995, Kiribati moved all its islands to the same side. Its Line Islands now use UTC+14, the earliest time on Earth.
- Samoa wanted to match the business week of its biggest trading partners, Australia and New Zealand. At midnight at the end of Thursday 29 December 2011, it jumped straight to Saturday 31 December. Friday 30 December 2011 never happened in Samoa.
Because of these moves, the earliest and latest clocks on Earth are 26 hours apart, not 24. For a full guide to the line's twists and turns, read the International Date Line explained.
Why do we change our clocks in spring and autumn?
In many countries, clocks move forward one hour in spring and back one hour in autumn. This is daylight saving time: "spring forward, fall back".
The idea is to move an hour of daylight from the early morning, when most people are asleep, to the evening, when they can use it. Germany was the first country to do this nationwide, on 30 April 1916, during the First World War, to save coal used for lighting.
Today, countries in the European Union change their clocks on the last Sunday of March and the last Sunday of October. Most of the United States and Canada switch on the second Sunday of March and the first Sunday of November. Not everyone joins in. Hawaii and most of Arizona keep the same time all year, and so do China, India and Japan.
Daylight saving also explains why the London–Tokyo gap shrinks from nine hours to eight in summer: London moves its clocks forward, but Tokyo does not. When you are ready to test what you know about the countries behind all these zones, try the world geography quiz.