What is a time zone?

A time zone is an area of the world where everyone sets their clocks to the same time. Inside one time zone, it's the same time on every clock, whether you're in a big city or a small village a few hundred miles away.

Each time zone is described by how far ahead of or behind a shared reference time it is. That reference is UTC (Coordinated Universal Time), the world's agreed standard time.^[1] For example, at 12:00 noon UTC on a day in January, the clocks read:

• 12:00 in London, which is on UTC in winter
• 7:00 in New York, five hours behind (UTC−5)
• 17:30 in New Delhi, five and a half hours ahead (UTC+5:30)
• 21:00 in Tokyo, nine hours ahead (UTC+9)

It's the same moment everywhere. Only the number on the clock is different.

For most of history, every town kept its own time by the sun. Noon was when the sun was highest in the sky, so noon came a little earlier in towns to the east and a little later in towns to the west. In England, Bristol ran about ten minutes behind London.^[2] A clock on Bristol's Corn Exchange still has two minute hands, one for each.^[3]

That worked when the fastest way to travel was a horse. It stopped working once railways and the telegraph arrived. Train timetables had to list dozens of local times, and a few minutes' confusion could put two trains on the same track.

So railways grouped towns into shared zones. Britain's railways adopted Greenwich time in 1847.^[2] On November 18, 1883, the railroads of the United States and Canada switched to standard zones, four of them across the continental United States.^[4] In 1884, an international conference in Washington, D.C. chose the line through Greenwich, in London, as the starting point for measuring longitude, and over the following decades most countries set their clocks relative to it.^[2]

The Earth turns once every 24 hours, which is 15 degrees of longitude every hour. Cut the globe into 24 slices of 15 degrees each and you get one time zone per hour, with the time in neighboring slices one hour apart.

That's the idea, but real time zones don't follow straight lines. Countries decide their own time, so zone borders bend around countries, states and islands, and some places pick offsets that aren't whole hours:

• China is wide enough for about five zones, but the whole country uses one time, UTC+8.^[5]
• India uses UTC+5:30, and Nepal uses UTC+5:45.^[6]
• Newfoundland, in Canada, is half an hour ahead of its neighbors, at UTC−3:30.^[6]
• France has more time zones than any other country, 12 in all, because of its overseas territories.^[7]

Counting all the half-hour and quarter-hour offsets, there are nearly 40 different offsets in use, running from UTC−12 to UTC+14.^[8] For every zone and its current offset, see the list of time zones.

You'll see the same time zone written in a few different ways:

• As an offset, like UTC−5 or UTC+9. This tells you exactly how far a clock is from UTC right now.
• As a name or abbreviation, like Eastern Standard Time (EST) or Central European Time (CET). These are easy to read but can be ambiguous: IST means India Standard Time, Irish Standard Time or Israel Standard Time, depending on who's writing.
• As a region ID, like America/New_York or Asia/Tokyo. Phones and computers use these. Each one points to a full history of that place's offsets and clock changes, kept in the IANA time zone database.^[6]

When you're arranging something with another person, a city name is often the clearest choice. "3 PM New York time" can't be misread, and it stays right all year, even when the offset changes.

Many places move their clocks forward an hour in summer and back in winter. This is daylight saving time. New York, for example, is UTC−5 in winter and UTC−4 in summer. London is on UTC in winter and UTC+1 in summer.

Not everyone does this. Most of Asia and Africa keep the same time all year, and in places that do change, the dates aren't the same everywhere: the United States changes its clocks on the second Sunday in March and the first Sunday in November,^[9] while Europe changes on the last Sundays of March and October.^[10] So for a few weeks each year, the gap between New York and London is four hours instead of five.

This is why a time zone is more than a single offset. It's a place's whole set of rules about what its clocks say, and those rules can change when a government decides to change them.^[6]

Because time zones are ahead of and behind each other, it can be a different day in two places at once. When it's 9 PM on Monday in Los Angeles, it's already Tuesday afternoon in Sydney.

The day starts first in the Pacific, just west of the International Date Line, which runs roughly along the 180° line of longitude, halfway around the world from Greenwich. Crossing the line going west, you skip ahead a day; going east, you repeat one. Countries can move to either side: Samoa skipped December 30, 2011 entirely when it switched sides to share a working week with Australia and New Zealand.^[11]

To see how offsets, borders and local rules combine to set the time in a place, read how time zones work. If you've wondered why some places say UTC and others say GMT, read UTC vs GMT.

And when you need the time somewhere right now, open the world clock. It shows the live time in any city next to your own, with daylight saving changes already worked in.

1. 1.NIST, UTC(NIST) time scale. What UTC is and who keeps it.
2. 2.Royal Museums Greenwich, Greenwich Mean Time (GMT). Local mean times such as Bristol's, railway time in 1847, and the 1884 conference.
3. 3.Atlas Obscura, Corn Exchange Dual-Time Clock. The Bristol clock with two minute hands.
4. 4.Library of Congress, Today in History: November 18. The railroads' standard time zones of November 18, 1883.
5. 5.Guinness World Records, Largest country in one time zone. China's single time zone.
6. 6.IANA, Time Zone Database. Offsets for India, Nepal and Newfoundland, region IDs, and how zone rules change over time.
7. 7.Guinness World Records, Country with the most time zones. France's 12 time zones.
8. 8.Wikipedia, List of UTC offsets. The number of offsets in use and the UTC−12 to UTC+14 range.
9. 9.U.S. Department of Transportation, Daylight Saving Time. US clock change dates.
10. 10.EUR-Lex, Directive 2000/84/EC on summer-time arrangements. EU clock change dates.
11. 11.The Christian Science Monitor, Samoa skips Friday in leap across international dateline. Samoa skipping December 30, 2011.