How time zones work, and why there are more than 24 of them

Before zones: every town kept its own noon

Earth turns once in about 24 hours, which is 360 degrees of longitude. Divide one by the other and you get the number every time zone is built on: the sun's position shifts by 15 degrees of longitude per hour, or one degree every four minutes. Two towns 15 degrees apart see solar noon an hour apart; two towns one degree apart see it four minutes apart.

Until the 1840s that was simply how clocks worked. Each town set its clocks to its own solar noon, so Bristol ran about ten minutes behind London and nobody minded, because nothing moved fast enough for ten minutes to matter. Railways changed that. A timetable with a different "noon" at every station is unusable, so British railway companies adopted Greenwich time across their networks in the 1840s, and North American railroads switched to a system of hour-wide standard zones on November 18, 1883.

The following year, the International Meridian Conference in Washington recommended the Greenwich meridian as the zero line of longitude. That is why the reference offset is measured from a suburb of London rather than from Paris or Washington: it was the meridian most of the world's shipping charts already used.

An offset is just a number of hours from UTC

Modern zones are defined as offsets from Coordinated Universal Time (UTC). New York is UTC-5 in winter, Lagos is UTC+1 all year, Tokyo is UTC+9. The ideal zone is a 15-degree slice centred on a meridian that is a multiple of 15 degrees, so that solar noon falls close to 12:00 clock time everywhere inside it.

Reality is messier. Offsets in use today run from UTC-12, which is kept only by two uninhabited US islands, through UTC-11 in American Samoa and Niue, all the way to UTC+14 in Kiribati's Line Islands. That is a 26-hour spread, which is how one planet can be on three calendar dates at the same moment. About three dozen distinct offsets are in use at any one time, because several countries sit on half-hour and quarter-hour offsets and because daylight saving shifts some zones for part of the year.

You can see all of this on the live globe: switch it to the Time zones view and every region is coloured by its current offset, with a legend listing every offset in force at that moment.

Why the lines are not straight

A zone boundary is a political decision, not an astronomical one. Governments choose offsets for trade, for alignment with a neighbour, or for national unity, and the result is a map that looks nothing like fifteen-degree stripes.

• China spans about 60 degrees of longitude, enough for five hour-wide zones, but has used a single clock, UTC+8, since 1949. In the far west that puts solar noon at around 2 PM, and many people in Xinjiang informally keep a second, unofficial clock two hours behind. The China page shows the official zone and notes the unofficial one.
• France holds the record for the most zones of any country, twelve, once you count its overseas territories from French Polynesia to New Caledonia. Metropolitan France itself sits on UTC+1, a Central European offset, even though Paris is geographically closer to the Greenwich meridian than to the 15-degree line.
• Spain is west of London yet keeps Central European Time, a choice made in 1940. Solar noon in Madrid falls at about 1:15 PM in winter and 2:15 PM in summer, which is part of why Spanish mealtimes look late to visitors. The equation of time guide walks through that calculation.
• Russia uses eleven zones, and has redrawn them several times in the last two decades. The United States uses nine standard zones once its territories are counted, from Puerto Rico in the Atlantic to Guam in the Pacific.
• India uses one offset, UTC+5:30, for a country that spans about 30 degrees of longitude, so sunrise in the far north-east comes well over an hour before sunrise in the west.

The database your devices actually trust

None of this would work if every program kept its own list of rules. Instead almost every operating system, browser and programming language reads the IANA time zone database, often called the tz database or zoneinfo. It records, for every zone, the full history of offsets and daylight saving rules, so a date in Moscow in 2012 is converted with the rules Moscow used in 2012, not the ones it uses today.

Zones in that database are named after a representative city, such as Europe/London or America/Argentina/Buenos_Aires, rather than after an abbreviation, because abbreviations are ambiguous and cities are not. The database is updated several times a year whenever a government announces a change, sometimes with only weeks of notice.

World Time Lab ships no time zone data of its own. Every clock on this site reads the IANA database built into your browser, which is why a freshly updated device and a stale one can disagree by an hour in the weeks after a rule change. The methodology page explains that limit in full.

What this means when you convert a time

1. Name the place, not the abbreviation. "3 PM in Chicago" has one meaning; "3 PM CST" has at least three.
2. Check the date. A zone's offset can differ by an hour between July and January, and the changeover dates differ by country. Every country page lists the exact dates the clocks change.
3. Convert through UTC. Subtract one offset to reach UTC, then add the other. The converter does this for any number of clocks at once and shows the local date beside each time, so you can see when a conversion crosses midnight.

See it on the site

• Live world clock globeSwitch to the Time zones view to see every offset in force right now, coloured on a real map.
• Time by countryEvery zone a country uses, with the dates its clocks change in the current and next year.
• Conversion chartsHour-by-hour tables for common zone pairs, computed for dated examples rather than assumed.