Home / Guides / How time zones work
How Time Zones Work: UTC, Offsets, Half-Hour Zones and the Date Line
Before the late 1800s, every town kept its own time, set by the sun: noon was when the sun was highest overhead. That worked until railroads made it possible to cross hundreds of miles in a day. A train timetable listing dozens of slightly different local times was a mess, so in 1883 the US and Canadian railroads agreed on a handful of standard time zones. The rest of the world followed over the next few decades.
The basic idea
The Earth turns 360 degrees in 24 hours, which is 15 degrees per hour. In theory, then, the world splits into 24 zones, each 15 degrees wide and one hour apart. In practice, zone borders follow countries and states rather than straight lines, because nobody wants a time zone boundary running through the middle of a city.
UTC and offsets
Every time zone is described by its distance from UTC, Coordinated Universal Time, the world's reference clock. New York in winter is UTC−5, which means it is five hours behind UTC. Tokyo is UTC+9. To find the difference between two places, subtract one offset from the other: Tokyo (+9) minus New York (−5) is 14 hours.
UTC itself doesn't change for daylight saving time, which is why pilots, programmers and scientists use it. For more on the difference between UTC and GMT, see UTC vs GMT.
Abbreviations can be misleading
Letters like EST, CST and IST are handy, but they are not unique and they hide daylight saving changes. "EST" technically means Eastern Standard Time (UTC−5), but people often use it in summer when New York is really on EDT (UTC−4). "IST" can mean India, Ireland or Israel. When it really matters, write the city instead: "3 PM New York time" is impossible to misread. Our time zone list shows what each abbreviation means.
Not every zone is a whole hour
A few places use offsets of 30 or 45 minutes:
- India uses UTC+5:30 for the whole country.
- Newfoundland in Canada is UTC−3:30 (−2:30 in summer).
- Iran is UTC+3:30, and parts of Australia (South Australia and the Northern Territory) are 30 minutes off their neighbors.
- Nepal is UTC+5:45, and New Zealand's Chatham Islands are UTC+12:45.
These exist because a country chose the time that best matched the sun over its main population, rather than rounding to the nearest hour.
Big countries, few zones
The size of a country doesn't decide how many zones it uses. The mainland United States uses four, and Russia uses eleven. China, which is about as wide as the continental US, uses just one: all of China runs on Beijing time (UTC+8). In the far west of China the winter sun doesn't rise until well after 9 AM, and many people there keep an unofficial local time two hours behind.
The International Date Line
If you keep going east and adding hours, eventually you meet people who keep going west and subtracting them. The meeting point is the International Date Line, which runs roughly along 180° longitude in the Pacific. Cross it heading west and you skip forward a day; cross it heading east and you repeat a day.
The line zigzags around island countries so that each country keeps one date. Kiribati moved it in 1995 so that all of its islands share a date, which is why its Line Islands are on UTC+14: the first place on Earth to reach each new day, and the first to celebrate New Year. American Samoa, only a few hundred miles away, is on UTC−11, nearly a full day behind.
Seeing it for yourself
The easiest way to understand time zones is to watch them. The 3D time zone map shows every zone on a globe with the line between day and night, and the world clock lists the current time in hundreds of cities.
Quick answers
How many time zones are there?
There are 24 one-hour zones in theory, but because of half-hour and 45-minute offsets and places as far as UTC+14 and UTC−12, more than 35 different local times are in use.
Why does India have a 30-minute time zone?
India chose UTC+5:30 so that one time would fit reasonably well across the whole country, which sits between two whole-hour zones.
Which place is first to reach a new day?
Kiribati's Line Islands, on UTC+14, are first to see each new day.
Spotted a mistake? Let us know and we'll fix it.