Time zones exist so that clocks everywhere roughly match the sun: noon should be around midday. In theory the world splits into 24 zones an hour apart, each 15 degrees of longitude wide. In practice, countries choose their own offsets for political, economic and geographic reasons, which is why some places are 30 or 45 minutes off the hour, China runs on a single time, and the lines on a time zone map zigzag wildly.
How time zones began
Until the 19th century, every town kept its own local time by the sun. Railways made that unworkable: a train timetable needs one clock. British railways adopted London time in the 1840s, and in 1883 the railways of the United States and Canada switched to a handful of standard zones. The next year, the International Meridian Conference in Washington chose Greenwich as the prime meridian, the zero line from which longitude, and in time the world's time zones, are counted. Today the reference is UTC, Coordinated Universal Time, kept by atomic clocks.
The half-hour and 45-minute zones
| Place | Offset from UTC (January) |
|---|---|
| India | UTC +5h 30m |
| Nepal | UTC +5h 45m |
| Iran | UTC +3h 30m |
| Afghanistan | UTC +4h 30m |
| Myanmar | UTC +6h 30m |
| Newfoundland, Canada | UTC −3h 30m |
| South Australia | UTC +10h 30m |
| Northern Territory, Australia | UTC +9h 30m |
India's single offset of five and a half hours is a compromise across a country nearly 30 degrees of longitude wide. Nepal, at 5 hours 45, sets itself apart from India. A few small places go further: New Zealand's Chatham Islands sit at 12 hours 45 (13 hours 45 in summer).
Big countries, one clock
China spans about five time zones' worth of longitude but uses one, Beijing time, so in the far west the sun can rise as late as 10 AM. Russia, at the other extreme, has eleven zones. France, counting its overseas territories, has the most of any country: twelve.
Daylight saving
Many countries move their clocks forward an hour in summer, to shift daylight into the evening. Most of North America and Europe do; most of Asia, Africa and South America don't. They also change on different dates: the US and Canada move in March and November, Europe in late March and late October, and southern-hemisphere countries like Australia and New Zealand the other way round. That's why the gap between two cities can change during the year, sometimes for just a few weeks.
The International Date Line
Go far enough east and you reach the point where the clocks are 12 hours ahead of UTC, and just across it, 12 behind: the date changes there. Our post on the International Date Line explains how it works and why it zigzags.
Convert any time
The time zone converter handles all of this, daylight saving and half-hour zones included, between any two cities with an airport, and the world clock shows the time now in as many as you like. In Flight, crossing time zones is part of every focus flight, and the landing screen tells you the local time.