How time zones work
Why it is already evening in Manila while London has its morning, where half-hour zones come from and how one country keeps a single clock across five solar zones. Turn the time on the globe and see where it is noon right now and where it is already tomorrow.
A day on the globe
The Earth turns 15 degrees an hour, and noon sweeps around the planet with it: at the equator it travels at about 1,670 kilometres an hour. If everyone lived by the Sun, the clocks in every town would show a different time. So the world agreed on time zones: inside a zone all clocks show the same time, and neighbouring zones differ by a whole number of hours. Almost always.
The chapter numbers of this article are Greenwich times: the article lives through one day. The globe below is live. Turn the time and watch the line between day and night creep west and the noon line move, while the coloured bands show how time is really divided.
Right now 37 different offsets from UTC are in use on the inhabited Earth, from UTC−11 to UTC+14. The westernmost and easternmost zones are 25 hours apart, so at any one moment it is still yesterday somewhere and already tomorrow somewhere else.
Every town had its own noon
Two hundred years ago every town set its clocks by the Sun: noon was when the Sun stood highest. Each degree of longitude shifts that noon by 4 minutes. Bristol lies 2.6 degrees west of London, and Bristol's clocks ran 10 minutes behind London's. The clock on Bristol's Corn Exchange still has two minute hands: one for London and one for Bristol.
As long as people travelled on horseback, nobody minded. Railways and the telegraph changed that: a timetable with dozens of local times became a puzzle. In 1847 British railways switched to Greenwich time, and on 18 November 1883 the railways of the United States and Canada introduced time zones; on that "day of two noons" clocks in many towns had to be reset in the middle of the day. In 1884 an international conference in Washington chose the meridian through the Greenwich observatory as the prime meridian. Time zones are still counted from it.
Clocks versus the Sun
In an ideal world, time zones would be neat 15-degree slices. In the real one, zone boundaries follow the borders of countries and regions, and every country decides for itself which clock to keep. That is why clocks disagree with the Sun almost everywhere.
China stretches across more than 60 degrees of longitude, yet since 1949 it has kept a single Beijing time. In Kashgar, in the far west, the Sun is highest at around three in the afternoon. Spain has kept Central European Time since 1940, although nearly all of it lies west of Greenwich, so in summer the Sun over Madrid is highest after two in the afternoon. And some places split the hour in half or even into quarters: India is on UTC+5:30, Nepal on UTC+5:45, New Zealand's Chatham Islands on UTC+12:45, and UTC+13:45 in summer.
Where tomorrow begins
The date on the planet changes at two boundaries. The first is midnight, which runs west behind the Sun. The second is the International Date Line in the middle of the Pacific: cross it from west to east and you step into yesterday. The line roughly follows the 180th meridian but bends around island nations so that each of them keeps a single date.
Countries have crossed it more than once. The Philippines skipped 31 December 1844 to share a calendar with its Asian neighbours. In 1995 Kiribati moved the line east, and its Line Islands now keep UTC+14, the first place to see the new year. Samoa skipped 30 December 2011. And every day from 10:00 to 11:00 Greenwich time, three different dates are in use at once even on inhabited islands: it is still yesterday in American Samoa and already tomorrow on Kiritimati.
Daylight saving time
In summer the Sun rises early, and part of the bright morning is slept away. The idea of daylight saving time is to move the clocks an hour forward in spring so that the light hour goes to the evening. Germany and Austria-Hungary were the first to do it, in 1916, to save coal during the war.
Today clocks change mostly in Europe and North America, and on different days. In 2026 Europe returns to standard time on 25 October, while the United States does so on 1 November. For a whole week London and New York are not five hours apart but four. Most countries, including China, Japan, Russia and the Philippines, never change their clocks.
How your phone knows the time
Phones and computers keep time in Coordinated Universal Time, UTC, and work out local time from the IANA time zone database. It holds hundreds of zones with names like Asia/Manila or Europe/Warsaw, together with the whole history of their rules: when a country moved to another zone, when it introduced or abolished daylight saving time. Governments change the rules several times a year, and the database is updated with them. In 2026 alone, British Columbia, Alberta and Manitoba stopped changing their clocks and stayed on summer time, and Morocco went back to Greenwich time after eight years on UTC+1.
That is why programmers don't trust abbreviations. PST is both Pacific Standard Time in California, UTC−8, and Philippine Standard Time, UTC+8: sixteen hours apart. Only zone names and offsets from UTC are reliable.
