What are time zones?

What is a time zone?

A time zone is a region that has agreed to observe the same civil time. The agreed time is defined as a fixed offset from Coordinated Universal Time — the international time standard maintained by atomic clocks — and may be modified seasonally by daylight saving time. Wikidata defines the term tersely as "region on Earth that has a uniform standard time for legal, commercial, and social purposes."³

The same instant in time has the same UTC value everywhere on Earth. What differs by time zone is the local label that instant carries: 12:00 UTC is 13:00 in London on a winter day, 14:00 in Berlin, 21:00 in Tokyo, and 04:00 in San Francisco. The civil time in any place equals UTC plus that place's offset, plus a daylight-saving adjustment if the zone is currently observing one.

In practice, "time zone" can mean three slightly different things, and software treats them as separate:

• The offset. A pure number — UTC+5:30, UTC−4 — that says how far the local clock is from UTC. Adequate for a single instant; inadequate for any computation that crosses a daylight-saving transition.
• The IANA zone identifier. A name like Asia/Kolkata or America/New_York that points to a rule set: the offset now, the offset before each historical change, the daylight-saving rules, and the dates of every transition. This is the operational primary key used by Linux, macOS, the JavaScript Intl API, the Java java.time package, the Python zoneinfo module, and almost every other system that handles dates.¹
• The everyday name. "Indian Standard Time", "Eastern Time", "Central European Summer Time" — the names people actually use in conversation. They are derivative of the IANA identifier and the season.

The offset is what most people picture when they hear "time zone". The IANA identifier is what makes a calendar invite that survives a daylight-saving transition possible.

Why do time zones exist?

Until the second half of the 19th century there were no time zones. Every town kept its own mean solar time: a clock set so that 12:00 noon was the moment the Sun crossed the local meridian, plus or minus a few minutes for the equation of time. Two towns thirty miles apart had clocks that disagreed by about two minutes, and nobody minded, because no one could travel fast enough or communicate quickly enough for the difference to matter.

Railroads and the telegraph broke that. By the 1860s a single North American railroad might pass through a dozen incompatible local times in a day's run, and the printed timetable in one city would not agree with the printed timetable in another. The first response was the railroad's own solution: standard times set by the rail companies and posted in their own stations, sometimes in addition to the local town clock and sometimes in place of it. The second response was a coordinated national one. On Sunday, 18 November 1883 — a date later remembered as "the day of two noons" — every railroad in the United States and Canada simultaneously reset its clocks to one of five standard zones, on a telegraphed signal sent from the Allegheny Observatory in Pittsburgh at noon on the 90th meridian. The proposal was the work of William F. Allen, secretary of the railway industry's General Time Convention, who had spent the preceding two years assembling a five-zone scheme that the railroads would actually accept.¹³

The world followed. The 1884 International Meridian Conference in Washington formalised a global reference point against which all standard zones could be defined. National adoption took the rest of the century and a few decades after that — Britain had codified Greenwich Mean Time as legal time as early as 1880, the United States made the railroads' five zones civil law only in 1918, France switched from Paris time to Greenwich-derived time in 1911 — but the railway demonstration had already settled the principle.¹³

What did the 1884 Washington conference decide?

The International Meridian Conference met in Washington, D.C. in October 1884. Twenty-six nations sent forty-one delegates. Over three weeks the conference passed seven numbered resolutions and adjourned. The two that mattered for civil timekeeping were:⁶

• Resolution 2, adopted on 13 October by a vote of 22 to 1 (the Dominican Republic against; France and Brazil abstaining): "the meridian passing through the centre of the transit instrument at the Observatory of Greenwich" was adopted as "the initial meridian for longitude" — the prime meridian against which every other longitude on Earth would be measured.⁶
• Resolution 4: a "universal day" would be adopted "for all purposes for which it may be found convenient, and which shall not interfere with the use of local or other standard time where desirable." Resolution 5 specified that the universal day was a mean solar day beginning "at the moment of mean midnight of the initial meridian" — the same instant marked, today, as 00:00 UTC.⁶

What the conference did not do was adopt time zones. The Canadian engineer Sandford Fleming, who had been campaigning for a worldwide system since his 1876 paper Terrestrial Time, had proposed a scheme of twenty-four hourly zones lettered A through Y (omitting J). The conference noted his proposal and declined it: standard zones were "outside its purview" and a matter for individual nations.¹⁴ The hourly grid of zones spread anyway, country by country, over the following half-century — and where a country chose a non-integer offset or split its territory across multiple zones, that was the country's choice to make.

How is a time zone defined today?

A time zone today is two things together: a standard offset from UTC, and a rule for whether and when that offset changes seasonally. The civil time at any place equals UTC plus the standard offset, plus a daylight-saving correction if the zone is observing one at that instant. Both pieces are set by the sovereign government of the place, can change by decree, and frequently have.

The standard offset is conventionally written as UTC±HH:MM. The vast majority of zones use whole hours: UTC, UTC+1, UTC+2, and so on through UTC+12, plus their negative counterparts. A small minority use thirty-minute or forty-five-minute increments, almost always for historical reasons that long predate any modern coordination — Indian Standard Time at UTC+5:30 has been the same offset since the colonial Indian government set it in 1906, and Nepal Standard Time at UTC+5:45 was set so that local noon at the Royal Observatory at Kathmandu would be exactly 12:00.⁵

Daylight saving is a per-zone seasonal modifier on top of the standard offset. Some zones observe it, some never do, and many have adopted and abandoned it more than once over the past century. The history of every zone's daylight-saving rules — when they took effect, which weekend the clock changes fall on, whether the rule was repealed, when — is part of the zone's full definition; that history is what the IANA database records, and what software needs to convert between civil time and UTC correctly across any historical date.

What is the IANA Time Zone Database?

The IANA Time Zone Database — also called the tz database, the zoneinfo database, or just tz — is a public-domain compilation of the world's time-zone history and rules, maintained as a single set of plain-text source files plus the C code that compiles them into the binary format every Unix-derived system reads. The database describes itself as "the history of local time for many representative locations around the globe."¹

The database was started in 1986 by Arthur David Olson, then a researcher at the U.S. National Institutes of Health, and maintained from his account at NIH for the next twenty-five years. It is the single piece of timekeeping infrastructure most computing depends on without naming. Linux, macOS, BSD, the JavaScript Intl API, Python's zoneinfo module, Java's java.time, PostgreSQL, and Windows by way of an ICU mapping all read the same source data, recompiled to whatever each platform's native binary format is. A volunteer mailing list reviews proposed updates — a country announcing a daylight-saving change, a corrected historical date, a previously unrecorded ambiguity — and a designated coordinator releases new versions on the order of half a dozen times a year. The current coordinator is Paul Eggert; the current release, as of April 2026, is 2026b.¹

The database transitioned from Olson's personal stewardship to formal institutional maintenance in 2011. The triggering event was a copyright lawsuit filed by an astrology software company against Olson and a colleague in October of that year; the suit was dropped within weeks, but the episode underscored how vulnerable the project was to its single maintainer being unable to continue. ICANN, which delegates the IANA function on behalf of the global internet community, agreed to take over administrative responsibility on 14 October 2011. The procedures were codified in a short Internet standards document — Internet Engineering Task Force Best Current Practice 175, published as RFC 6557 in February 2012 — which establishes how the database is updated, who selects the coordinator, and how the community appeals decisions.⁷⁸

Naming conventions

Each zone in the database has a canonical name in the form Area/Location, where Area is a continent or ocean (America, Europe, Asia, Africa, Australia, Antarctica, Indian, Pacific, Atlantic, plus the special Etc) and Location is conventionally the largest city in the region the rule set applies to.¹⁵ Examples in heavy daily use include America/New_York, America/Los_Angeles, America/Chicago, Europe/London, Europe/Berlin, Asia/Tokyo, Asia/Kolkata, Asia/Shanghai, Australia/Sydney, and Pacific/Auckland.

The naming convention is deliberate and pragmatic. Cities are unambiguous; countries are not (a country's borders change, but the City of London does not move), and no political body has any authority to dictate names to the global timekeeping community. Names that turn out to be wrong are kept as alias links to the corrected name, so that systems that have stored old identifiers continue to work. Asia/Calcutta and Asia/Kolkata, for example, both resolve to the same rule set; the latter is the canonical name and the former is a link kept for compatibility.

Coverage scope

The database makes its strongest guarantees about civil time after the start of the Unix epoch — 1 January 1970 at 00:00 UTC — and is best-effort before that. The database's design notes are unusually explicit about this:

If all clocks in a region have agreed since 1970, give them just one name even if some of the clocks disagreed before 1970, or reside in different countries or in notable or faraway locations.²

The companion machine-readable file zone1970.tab is the operational table of zones since the epoch, keyed by ISO 3166 country code. Earlier history is preserved where it can be reconstructed — the database has rules for many zones reaching back to the 19th century — but the maintainers warn that it "is not designed for and does not suffice for applications requiring accurate handling of all past times everywhere."² Software that needs to be correct in 2026 about a civil time in 2025 is a good fit; software that needs to be correct about a civil time in 1925 should expect occasional surprises.

How many time zones are there?

About thirty-eight different standard offsets are in routine use today, spanning the twenty-six hours from UTC−12:00 to UTC+14:00. The two extremes are the Howland and Baker Islands — uninhabited US Pacific possessions — at UTC−12, and the Line Islands of Kiribati at UTC+14. Every other inhabited place on Earth is on some intermediate offset.⁴⁵

The IANA database currently contains several hundred named zones, but most of them describe the same offset and rules — they are kept separate so that historical pre-1970 differences are preserved, or so that future divergence can be encoded without renaming. The number of live distinct rule sets is far smaller. Five zones cover the contiguous United States; one zone covers all of China, even though the country is wide enough geographically to fall into five hourly zones; one zone covers all of India; the European Union runs on three.

A handful of jurisdictions sit on non-integer offsets. The thirty-minute offsets — UTC+5:30 in India and Sri Lanka, UTC+3:30 in Iran, UTC+4:30 in Afghanistan, UTC+6:30 in Myanmar, UTC−9:30 in the Marquesas, UTC−3:30 in Newfoundland, UTC+9:30 in central Australia — are mostly historical, set in the late 19th or early 20th century to bring the local civil clock close to local mean solar time, and never adjusted since. The forty-five-minute offsets are rarer still: UTC+5:45 in Nepal and UTC+12:45 in the Chatham Islands of New Zealand, both with similar local-mean-time origins.⁵

When have countries unilaterally changed their offset?

Time zones are a sovereign matter. A national government can move its civil clock by any amount, in either direction, on whatever date it pleases, and the IANA database simply records the change. The most striking cases of the past three decades:

Kiribati, 31 December 1994. The Republic of Kiribati straddles the equator and the 180° meridian: its three island groups span 4,800 kilometres east to west. Until 1994 the western Gilbert Islands were on UTC−11 and UTC−10, and the eastern Phoenix and Line Islands were on UTC−11 and UTC−10 — placing the eastern islands on the far side of the International Date Line from the country's capital. The 1994 decree shifted both eastern groups across the line: the Phoenix Islands to UTC+13 and the Line Islands to UTC+14. The country's territory has been on a single side of the date line ever since, at the cost of a date line that bulges 1,000 kilometres east around the country.⁹

Samoa, 30 December 2011. Samoa, like Kiribati, sat on the wrong side of the date line for trade with its largest partners. New Zealand, Australia, and most of Asia were a calendar day ahead; the United States was a calendar day behind. At the end of Thursday 29 December 2011, Samoa skipped Friday 30 December entirely and woke up on Saturday 31 December as a UTC+13 country. The skip was widely covered as the day Samoa "lost" a Friday; it was, in practice, the same kind of date-line crossing as Kiribati's, but compressed into a single calendar discontinuity.¹⁰

Russia, 2011 and 2014. In March 2011 the Medvedev government extended that year's daylight-saving shift indefinitely: clocks went forward as usual in spring and never went back in autumn. Moscow time, which had been UTC+3 in winter and UTC+4 in summer, became UTC+4 year-round. The decision proved unpopular, and on 26 October 2014 the Putin government moved the country back the other hour and abolished daylight saving altogether. Moscow has been on UTC+3 year-round since. Russia today spans eleven hourly zones from UTC+2 in Kaliningrad to UTC+12 in the eastern Far East.¹¹

Venezuela, 2007 and 2016. In December 2007 President Hugo Chávez moved Venezuela from UTC−4 to UTC−4:30, restoring an offset the country had used between 1912 and 1965. In May 2016 President Nicolás Maduro reverted to UTC−4, citing a national electricity crisis: the half-hour shift had moved business hours later in the day, increasing evening lighting demand at a time of severely constrained hydroelectric supply.¹²

Turkey, 2016. Turkey ended seasonal time changes on 8 September 2016 and adopted permanent UTC+3, effectively keeping the country on its summer time year-round.¹⁶

North Korea, 2015 and 2018. On 15 August 2015 — the seventieth anniversary of Korean liberation from Japanese rule — North Korea moved its standard time from UTC+9 (which it had shared with Japan and South Korea since 1912) to UTC+8:30, calling the new zone Pyongyang Time. On 5 May 2018 the country reverted to UTC+9, this time framing the change as an act of inter-Korean alignment.¹⁷

How does daylight saving relate to time zones?

Daylight saving time is a property of the time zone, not a separate scale. Each IANA zone's rule set encodes whether daylight saving is observed, when the transitions fall, and how the rules have changed historically. Some zones — Iceland, almost all of Russia, Turkey since 2016, Iran since 2022, all of China, Japan, South Korea, India — never make a seasonal shift. Others, including most of Europe, most of North America, parts of South America, and parts of Australasia, do.

From the timekeeping system's point of view, a daylight-saving transition is simply a one-hour change in the offset on a specific instant. The standard offset is what defines the zone; daylight saving is a layer on top. The full discussion of why daylight saving exists, where it is observed, and how it interacts with civil clocks lives on the dedicated daylight saving time page.

Frequently asked questions