Time zones of the world

How time zones work

A time zone is a region whose clocks always agree. Three things define it: an offset from Coordinated Universal Time (UTC), the rules for when that offset changes for daylight saving time, and the history of both. The offset alone is not enough. New York and Panama are both on UTC−5 in January, but in July New York is on UTC−4 and Panama is still on UTC−5, so they are different time zones.

That is also why the clean picture of 24 one-hour slices of the globe never existed in practice. Under tzdata 2026f there are 39 different offsets in use, from UTC−11 to UTC+14, and 62 distinct combinations of offset and daylight saving rules. 101 of the 312 zones change their clocks during the year; the others keep one offset.

Offsets, abbreviations and zone names are three different things

“UTC−5”, “EST” and “America/New_York” are often used as if they meant the same thing. They do not:

• An offset (UTC−5) is a fixed number of hours and minutes. It is unambiguous for a single moment, and it is what ISO 8601 writes: 2027-01-15T09:00−05:00.
• An abbreviation (EST) is a name for an offset in a particular region and season. Abbreviations are not standardised and not unique: IST means India, Israel or Ireland, CST means US Central, China or Cuba. The tz database’s own documentation calls them ambiguous in practice and recommends numeric offsets instead.
• A zone name (America/New_York) identifies the full set of rules. It is the only one of the three that tells software what the local time will be next July or what it was in 1975.

The practical rule: write an offset for one moment, a zone name or city for anything recurring, and never an abbreviation alone when the audience is international.

The IANA time zone database

Every phone, computer and browser gets its time zone rules from the same source: the IANA time zone database, also called tzdata or the Olson database. It is maintained by volunteers, published by IANA and in the public domain. Release 2026f has 345 canonical zones and 253 links (aliases). Zone names follow the pattern Area/City, using the most populous city of the region (Europe/Paris, Asia/Kolkata, America/Argentina/Buenos_Aires). Since 2022 many country-specific names are links to another zone with identical clocks since 1970: Europe/Stockholm points to Europe/Berlin and Asia/Kuala_Lumpur to Asia/Singapore. They stay valid, and pages for them redirect to the zone they point to.

Governments change their rules often, sometimes with only weeks of notice. 2026 alone brought several: British Columbia moved to permanent UTC−7 in March, Alberta to permanent UTC−6 in June, the Northwest Territories in August and Manitoba to permanent UTC−5 at the end of October; Morocco moved to permanent UTC+0 in September; and Cuba decided not to fall back on November 1, 2026. Each change becomes a new tzdata release, and devices pick it up only when their operating system or browser updates. This site ships its own copy of the current release and uses it instead of the browser’s, so the clocks here are right even on devices that are behind.

Half-hour and 45-minute offsets

13 of the offsets in use are not whole hours: UTC−9:30, UTC−3:30, UTC−2:30, UTC+3:30, UTC+4:30, UTC+5:30, UTC+5:45, UTC+6:30, UTC+8:45, UTC+9:30, UTC+10:30, UTC+12:45 and UTC+13:45. The best known are India (UTC+5:30, the whole country), Nepal (UTC+5:45), Iran (UTC+3:30) and central Australia (UTC+9:30). A conversion between New York and India therefore always ends in :30, and one between India and Nepal is only 15 minutes.

Big countries, many zones, and one big exception

Counting only home territory, the countries with the most standard offsets are Russia (11), United States (6), Australia (5) and Canada (5). Overseas territories change the ranking: France keeps one zone at home but 11 offsets with its territories, the United States 10 and the United Kingdom 8. (Antarctica, which is not a country, has research stations on 8 different offsets.) China goes the other way: it spans about 60 degrees of longitude, enough for four or five hourly zones, but officially the whole country keeps Beijing Time, UTC+8. In Kashgar, in the far west, that clock runs almost three hours ahead of the sun. tzdata also has a second Chinese zone, Asia/Urumqi at UTC+6, because many Uyghur residents of Xinjiang keep this informal “Xinjiang time” while trains, planes and offices run on Beijing time.

Clock time and the sun

A zone’s offset is a political choice, so its clocks rarely match the sun. Mean solar time moves by four minutes for each degree of longitude: at 15° east it is exactly UTC+1, at 75° west exactly UTC−5. Every place page on this site gives the longitude of its main city and how far the clock runs ahead of or behind mean solar time; Vancouver’s clocks, for example, are more than an hour ahead since British Columbia kept UTC−7 all year. On top of that, the equation of time moves the real moment of solar noon by up to about a quarter of an hour either way through the year, because the Earth’s orbit is elliptical and its axis is tilted.

Daylight saving time: the gap and the repeated hour

When clocks go forward, an hour of local time does not exist; when they go back, an hour happens twice. In New York in 2027, 2:30 AM does not exist on March 14, 2027, and 1:30 AM happens twice on November 7, 2027, first in EDT and then, an hour later, in EST. Software has to decide what such a time means. The iCalendar standard used by calendar apps settles it: a skipped time is read with the offset from before the gap, so 2:30 AM becomes 3:30 AM EDT, and a repeated time means its first occurrence, in EDT (RFC 5545, §3.3.5). Every converter on this site shows when this happens. The full story, including the laws behind the dates, is on the daylight saving time page.

Mistakes an expert would flag

• EST in summer. New York is on EDT (UTC−4) from March to November; “EST” then means an hour earlier than the local clock. Use ET or the city.
• GMT for London in summer. The UK is on GMT only in winter; in summer it is on BST, UTC+1.
• The sign of Etc/GMT zones. Etc/GMT+5 is UTC−5: these POSIX-style names invert the sign.
• Storing an offset instead of a zone. “−05:00” is right today and wrong in July. Store the instant in UTC and the IANA zone of the event.
• Assuming one zone per country. The United States has 6 standard offsets and Arizona ignores daylight saving time; Australia has three mainland standard times, plus Lord Howe Island and the unofficial UTC+8:45 of Eucla (5 offsets in all), and its states choose their own daylight saving rules.
• Assuming everyone changes clocks on the same day. The US changes in March and November, Europe in late March and late October, Australia and New Zealand in October and April. For a few weeks each year, the gap between New York and London is four hours instead of five.