Time zones are the reason a phone call to another continent can wake someone up. This guide explains how they came about, how offsets from UTC work, and why the map of the world's time zones looks as strange as it does.
Why we need time zones
The Earth turns once in 24 hours, so the sun is highest in the sky at different times in different places. Until the 1800s every town set its clocks by the sun, which was fine when travel was slow. When railways connected the towns, the timetables had to deal with dozens of local times, and standard time became necessary.
In Norway the railway led the country to adopt a single standard time in 1895, based on the central European time zone.
Fifteen degrees per hour
The circumference of the Earth is divided into 360 degrees of longitude. Dividing by 24 hours gives 15 degrees for each hour. The simplest time zone system would therefore have 24 zones, each 15 degrees wide, with one hour between neighbors.
In 1884, delegates from about two dozen countries met in Washington, D.C. and chose the meridian through Greenwich in London as the zero line. Time zones are counted as hours ahead of or behind that line.
UTC and offsets
Today the reference is Coordinated Universal Time, abbreviated UTC, which is kept by atomic clocks. Each time zone is described as an offset from UTC, for example UTC+1 for Norway in winter and UTC−5 for New York in winter.
To convert, you add or subtract the difference between the two offsets. Oslo at UTC+1 and New York at UTC−5 are six hours apart in winter, and the difference is the same six hours in summer since both change, apart from a few weeks when they change on different dates.
When zones are not whole hours
Most zones are a whole number of hours from UTC, but a few are not. India uses UTC+5:30, Nepal UTC+5:45 and the Chatham Islands of New Zealand UTC+12:45. Newfoundland in Canada is at UTC−3:30. These half-hour and quarter-hour offsets are the result of political and historical choices.
The range of offsets in use is surprisingly wide. It goes from UTC−12 to UTC+14, a span of 26 hours, because some islands in the Pacific have chosen to be on the far side of the date line.
The international date line
Roughly along the 180th meridian runs the international date line. If you cross it going west, you add a day, and going east you subtract one. The line zigzags around countries and islands so that they stay on one date.
In 1995 Kiribati moved the line so that all of its islands share the same date, which put its eastern islands at UTC+14, making them among the first places in the world to see each new day.
Why the map looks odd
Borders follow politics more than longitude. China is as wide as five time zones but uses a single one, UTC+8, for the whole country. Russia has eleven. Spain is on central European time even though it is geographically in the same longitude as the United Kingdom. France has more time zones than any other country, if you count its overseas territories.
The effect is that the sun rises and sets at very different clock times within one zone. In western China it can still be dark at 9 am in winter.
The time zone database
Computers keep track of time zones with the IANA time zone database, which is updated several times a year as governments change their rules. Zones are named after a region and a representative city, such as Europe/Oslo or America/New_York, rather than abbreviations like CET, which are ambiguous.
Your browser uses this database, and so do the tools on this site, which is why they know about daylight saving changes without you having to enter them.
Daylight saving time and time zones
Many zones add an hour in summer, so a place has two offsets during the year. Oslo is UTC+1 in winter and UTC+2 in summer. When you convert between two places you need the offsets valid on that exact date, and the dates of the changes are not the same everywhere. In the United States the clocks change in March earlier than in Europe, and back in November later, which is why the difference between Oslo and New York is five hours for a short time in March and a week in October and November.
Practical tips
- When you schedule a meeting, give the time zone, or better, the time in the other person's city.
- Check the dates of the clock changes if you work across the Atlantic, because they differ by a week or two.
- For international events, UTC is the neutral choice.