How do time zones work? Why clocks differ around the world

Time zones work by dividing the Earth into regions that agree to keep the same clock time, each set a fixed number of hours (sometimes plus 30 or 45 minutes) ahead of or behind Coordinated Universal Time, or UTC. The idea comes from the Earth's spin: it turns 360 degrees in 24 hours, or 15 degrees an hour, so noon moves west at that rate. Where the zone lines actually fall is decided by governments, not geometry.

In short

• In theory, each zone is a 15-degree slice of longitude one hour wide; in practice, borders and politics shape them.
• Every zone is defined as an offset from UTC, an atomic time scale kept by laboratories around the world.
• Odd cases, such as half-hour zones, China's single zone and the date line, are choices countries made, and they can change by law.

Why are time zones needed at all?

Before the 19th century, every town kept its own time by the Sun. Noon was when the Sun stood highest, so a town just 1 degree of longitude east of another was 4 minutes ahead of it. Nobody minded while travel was slow. Railways and the telegraph changed that. A train timetable that had to list dozens of local times was a recipe for missed connections and, on single-track lines, collisions.

Britain solved it first, by making the whole country keep the time at Greenwich. The Railway Clearing House adopted Greenwich Mean Time in December 1847, and by the mid-1850s almost all public clocks followed it, according to Royal Museums Greenwich. GMT became Britain's legal time in 1880. A small country could get away with one time. A wide one could not.

How did the world settle on zones?

In North America, the railroads drew zones themselves. On 18 November 1883, station clocks across the United States and Canada were reset to five standard times proposed by the railway planner William F. Allen. Many towns saw noon twice that day, once by the Sun and again by the new clocks, which is why the Library of Congress calls it the Day of Two Noons. Congress didn't make the zones law until the Standard Time Act of 19 March 1918, which also brought in daylight saving time.

A year after the railroads acted, in October 1884, delegates from more than two dozen countries met in Washington, D.C., at the International Meridian Conference. They voted 22 to 1, with France and Brazil abstaining, to make the meridian through Greenwich the world's zero line of longitude. Part of the reason was practical: by the late 19th century, 72% of the world's commerce depended on sea charts that already used Greenwich, according to Royal Museums Greenwich. From that zero line, the world could count zones outward in whole hours.

How does UTC work, and how is it different from GMT?

GMT was time by the Sun at Greenwich. UTC, which took its present form in 1972 and replaced GMT as the world's reference, is time by atomic clocks. It is built from hundreds of clocks in laboratories around the world and ticks at a perfectly steady rate. The Earth's spin is slightly irregular, and over centuries the day is slowly getting longer, so UTC drifts from the Sun. To keep them within a second of each other, timekeepers added a leap second every so often, on average about every 18 months since 1972, according to NIST. There have been 27 in all, the last at the end of 2016. In 2022, the General Conference on Weights and Measures, the international body that oversees units of measurement, decided to let the gap grow larger by 2035. At its next meeting, in October 2026, it set the date: under its Resolution 5, UTC runs continuously, with no more leap seconds, from 20 May 2027, and the gap may grow to as much as 1 hour.

Every time zone is now written as an offset from UTC. New York in winter is UTC−5, Paris UTC+1, Tokyo UTC+9. In everyday speech, people in Britain still say GMT, and for clock purposes it's the same as UTC to within a second.

How do you work out the time somewhere else?

Turn the time into UTC, then into the other zone. Take a video call set for 9:00 in New York in the northern winter, when New York is UTC−5.

Place	Offset from UTC	Clock time for the call
New York	−5:00	9:00
UTC	0	14:00
London	0	14:00
New Delhi	+5:30	19:30
Kathmandu	+5:45	19:45
Tokyo	+9:00	23:00

Add 5 hours to get from New York to UTC, then add each place's offset. The same sum works in reverse. The catch is daylight saving time: in March, New York moves to UTC−4 and London to UTC+1 on different weekends, so the gap between them is briefly 4 hours instead of 5. That's why calendar software doesn't store offsets by hand. It relies on the tz database, a public record kept by IANA, the Internet Assigned Numbers Authority, of every zone's rules and changes, kept reliable back to 1970.

Why don't time zones follow straight lines?

Because countries choose their time, and most want one time across a country or a region. That gives three kinds of oddity.

Zones that don't fit their longitude. China spans more than 60 degrees of longitude, wide enough for five zones, but the whole country keeps one: UTC+8, Beijing's time. Spain, which lies mostly west of Greenwich, has kept Central European Time since 1940, when it set its clocks to match its neighbors to the east. The table below shows what that does to the Sun, using the average time of solar noon (the Sun's highest point) worked out from longitude at 4 minutes a degree.

Place	Longitude	Clock zone	Average solar noon, on the clock
London	0.1° W	UTC+0	about 12:00
Madrid	3.7° W	UTC+1 (UTC+2 in summer)	about 13:15 (14:15 in summer)
Kashgar, western China	76.0° E	UTC+8	about 14:56

Madrid is 3.7 degrees west of Greenwich, so its Sun peaks about 15 minutes after 12:00 UTC, which its clocks read as 13:15. Kashgar is 44 degrees west of the 120-degree meridian that Beijing time is based on, so its Sun peaks almost three hours after noon by the clock. The true time also shifts by up to about 16 minutes through the year, because the Earth's orbit is an ellipse and its axis is tilted.

Zones that aren't whole hours. India keeps UTC+5:30 so one time sits near the middle of the country. Nepal moved to UTC+5:45 in 1986, and New Zealand's Chatham Islands keep UTC+12:45. The offsets in use run from UTC−12 to UTC+14.

The date line. Go far enough east and you meet people who started the day before you. The International Date Line, roughly along 180 degrees, is where the calendar date changes. It isn't fixed by treaty, so countries move it. Kiribati, which straddled it, shifted its eastern islands to UTC+14 in 1995 so the whole country shares a date. Samoa jumped from Thursday 29 December 2011 straight to Saturday 31 December, skipping a day to line up its working week with Australia and New Zealand, its main trading partners.

What does daylight saving time do to the zones?

Daylight saving time doesn't change a place's zone; it changes the offset that zone uses for part of the year. Clocks go forward an hour in spring and back in autumn, shifting an hour of daylight from early morning to evening. Many countries don't use it at all, and those that do change on different dates. The United States and the European Union change on different Sundays, and countries near the equator, where day length barely changes, rarely bother. The reasons it began, and why it is argued over, are in why we have daylight saving time.

Time zones are a reminder that the Sun's position is physics but the clock is a human agreement. The physics comes from the Earth turning as it orbits within the solar system; the agreement is law, which is why the time in some places has changed several times within living memory. Measuring time precisely is one of science's quieter successes, and keeping time is still partly politics.

Check yourself

• Without looking back, explain why 15 degrees of longitude equals one hour.
• Why did railways, rather than astronomers, push the world into time zones?
• Why is solar noon in Madrid well after 12:00 on the clock?

Sources

• Greenwich Mean Time (GMT) — Royal Museums Greenwich
• The Day of Two Noons — Library of Congress, 2020
• Leap seconds FAQs — National Institute of Standards and Technology
• Time zone database — Internet Assigned Numbers Authority
• Resolution 4 of the 27th CGPM, on the use and future development of UTC — International Bureau of Weights and Measures, 2022
• Resolution 5 of the 28th CGPM, on the technical actions needed to ensure the continuity of UTC — International Bureau of Weights and Measures, 2026