Who decides when a new tzdata release is needed?

The maintainers decide, usually when a government announces a clock change. In2022, multiple jurisdictions abolished daylight saving, leading to a record number of releases.

How do I know which tzdata version my device is running?

On Linux, run `timedatectl`. On macOS, check the `/usr/share/zoneinfo` directory. On Windows, the version is embedded in the operating system build. For programming languages, consult the language's time zone library documentation.

Why does the database use city names instead of nation names?

City names are more stable. Nations can change their borders, their names, or their time zones. A city like London is unlikely to be renamed. The convention also handles nations that span multiple time zones: the US gets multiple cities, not a single `America/United_States`.

Does the database include every time zone that ever existed?

No. It covers every inhabited region from1970 onward. For earlier dates, the coverage is incomplete. The database focuses on the period that modern computers actually deal with.

What happens if a jurisdiction changes its time zone without warning?

The change is added to the database as soon as a reliable source is confirmed. The mailing list picks up the news, the maintainers write the rule, and a new release follows. The process can take days or weeks, depending on how quickly the evidence arrives.

Who should update their tzdata manually?

System administrators running servers that handle international scheduling, finance, or travel should watch for new tzdata releases. A missed update means a server will display the wrong time for any region that changed its clocks. For most users, automatic operating system updates are sufficient.

The IANA Time Zone Database: What It Is and How It Works

Every device that shows the correct local time is quietly consulting the same source: the IANA time zone database. It is not a treaty or an international standard. It is a set of plain text files, curated largely by volunteers, that records what each region of the world has done with its clocks.

A clock in Kathmandu runs 5 hours and 45 minutes ahead of UTC. A clock in Caracas ran 4 hours and 30 minutes behind for nine years, then shifted. A clock in Apia skipped 30 December 2011 entirely. None of these facts are guesses. They are lines in a text file maintained by a handful of volunteers and shipped to billions of devices. That file is the IANA time zone database.

It is more commonly called the tz database. Older documentation calls it the Olson database, after Arthur David Olson, the software engineer who began assembling it in the late 1980s. The project ships in two parts: tzdata, which holds the rules, and tzcode, the reference implementation that reads them. Between them they answer a question that sounds trivial and is not: what did, or will, a wall clock in this place read at this instant?

The data is deliberately historical. For any moment after 1 January 1970, the Unix epoch, the tz database aims to reproduce the local time that a clock in that region actually showed. That is why it can tell you not only that Berlin is currently one hour ahead of Coordinated Universal Time, but that a timestamp from 1979 in Berlin fell inside a daylight saving period that began on a date modern rules would not predict.

Who uses the IANA time zone database?

Most people never touch the database directly. Yet it runs underneath nearly every operating system in use today. When a phone shows the time in Tokyo, when a flight booking site converts departure times, when a server logs an event with a timestamp, all of them rely on the same tz data.

The database is the single source of truth for civil time. Without it, a program would have no way to know that Samoa skipped 30 December 2011, moving from UTC-11 to UTC+13. It would not know that Nepal uses a UTC+05:45 offset. It would not know that Morocco suspends daylight saving during Ramadan.

The history: Arthur David Olson to IANA

Arthur David Olson, a software engineer at the National Institutes of Health, started compiling time zone data in the late 1980s. The first public release came in 1986 or 1987. Olson's work was a response to a practical problem: early Unix systems handled time zones poorly, and there was no reliable source for the rules.

By the early 1990s, Paul Eggert had become the primary maintainer. Eggert, a computer scientist at UCLA, has overseen the database for over three decades. The project grew from a small collection of rules into the definitive record of global civil time.

In 2011, IANA (the Internet Assigned Numbers Authority) took over hosting the database. The move came after a legal challenge. Astrolabe Inc., a company that sold time zone software, sued Olson and Eggert in 2011, claiming copyright in the tz data. The case was dismissed in 2012. The lawsuit briefly took the files and the mailing list offline, but the database has been hosted under IANA stewardship ever since.

How zones are named: the Area/City convention

Zone identifiers follow an Area/Location pattern. The area is a continent or ocean: one of Africa, America, Antarctica, Arctic, Asia, Atlantic, Australia, Europe, Indian, or Pacific. The location is normally a large city inside the zone, not the nation itself.

America/New_York covers the US Eastern time zone. Asia/Kolkata covers all of India. Pacific/Auckland covers New Zealand. The city is chosen for stability: it is unlikely to be renamed or disappear. Nations that span multiple zones get multiple cities. The US alone has America/Chicago, America/Denver, America/Los_Angeles, and America/Anchorage.

The convention avoids the confusion of national labels. A nation can change its time zones or its borders; a city is more permanent. It also avoids the problem of three-letter abbreviations like EST, which can mean Eastern Standard Time, Eastern Summer Time, or something else entirely depending on the context.

What the database contains: rules, offsets, transitions

The tzdata files are plain text. They contain three things.

Zone lines define a zone's history. Each line gives a start date, a UTC offset, a rule name, and a comment. The comment might note when a nation changed its clocks.

Rule lines describe daylight saving transitions. They specify the year, the month, the day, the time of day, and the amount of change.

Link lines create aliases. US/Eastern is a link to America/New_York.

The database also includes a file called zone1970.tab, which lists each zone and its approximate geographic coordinates. This allows tools to guess a user's zone from their location.

For any moment after 1970, the database can calculate the UTC offset and whether daylight saving was in effect. For earlier dates, the coverage is less complete. The database does not track every local time before 1970; it focuses on the period that modern computers actually deal with.

How updates happen: the tz@iana.org process

Changes to the database are proposed and debated on a public mailing list: tz@iana.org. Anyone can submit a proposal, but the evidence must be solid. A government decree, a news report with an official source, or a translation of a legal notice is enough.

The typical process has five steps.

First, a national government publishes a decree changing its clocks. Second, a contributor reports it to the mailing list with the source document. Third, a new tzdata version ships, versioned by year and letter: 2023a, 2023b, 2024a, and so on. Fourth, operating systems, language runtimes, and browsers pull the release into their own updates. Fifth, your device applies the update, usually without anyone noticing.

The triggers for a new release are straightforward: a nation changes its daylight saving rules, a new zone is created, or a historical error is corrected. 2022 saw a record number of releases, driven by multiple jurisdictions abolishing or reinstating daylight saving.

The 2011 Astrolabe lawsuit and its aftermath

In 2011, Astrolabe Inc. sued Arthur David Olson and Paul Eggert, claiming that the tz database infringed on Astrolabe's copyright. The company sold a commercial product called "World Time Zone Database" and argued that the tz data was a derivative work.

The case drew widespread attention. The tz files were taken offline during the dispute. The Electronic Frontier Foundation defended Olson and Eggert. The case was dismissed in 2012, and the database returned to public access. IANA took over hosting the same year, ensuring the project had a stable institutional home.

The lawsuit had one lasting effect: it clarified that facts about time zones (the offsets, the rules, the dates) are not copyrightable. The database is factual, not creative. That principle protects the project from future claims.

How your devices get tzdata updates

The tzdata package flows into the world through operating system updates. Windows, macOS, and Linux distributions all ship updated versions. On Linux, the tzdata package is updated through the system package manager. On macOS, Apple bundles updates into its operating system releases.

Language runtimes also carry their own copies. Python's zoneinfo module, Java's tzupdater, and the time zone libraries in Go, Ruby, and PHP all rely on tzdata. Web browsers like Chrome and Firefox include compiled versions.

The weak link is the last step. A device that stopped receiving updates years ago is running an old copy. It will confidently display the wrong local time for any region that has changed its clocks since. That is why an old phone might show the wrong time for Egypt, which reinstated DST in2023 after a seven-year gap, or for Ukraine, which moved to permanent standard time in2024.

Common misconceptions about IANA zones

" The IANA database is the same as the time zone on my phone." Not quite. Your phone runs a compiled version of the database. The raw files are text. The phone's operating system converts them into a binary format for fast lookups.

" There is a standard time zone." There is not. Each jurisdiction sets its own rules. The IANA database simply records what they are.

" Three-letter abbreviations like EST are reliable." They are not. EST can mean Eastern Standard Time (UTC-5) in the US or Eastern Summer Time (UTC+11) in Australia. The IANA database avoids abbreviations entirely.

" The database is maintained by a government." It is not. It is maintained by volunteers, coordinated by Paul Eggert, with IANA as the formal steward.

" Once a time zone is in the database, it never changes." It changes constantly. A new tzdata release can ship any week. The rules for a jurisdiction can shift overnight.