IANA time zone database

The IANA time zone database, often called tz, tzdata or zoneinfo, is the shared record of local time around the world: every region's offset from UTC, its daylight saving rules, and how both have changed over time.^[1] Phones, computers, browsers and servers all read it to turn UTC into the time on a local clock, including TimeZonar, which uses the copy built into your browser.

The database is also known as the Olson database, after Arthur David Olson, who maintained it from the start.^[2] Today a volunteer TZ Coordinator decides what goes in, taking account of discussion on a public mailing list. The rules say the coordinator doesn't set time zone policy for anyone, but records what people in a place actually use.^[2]

The database moved to IANA, the body that also coordinates internet domain names and numbers, in October 2011. That happened after an astrology software company sued its maintainers over historical data. The company withdrew the suit in 2012, saying historical facts are no one's property.^[3]

• Area/Location: a continent or ocean, then a place in it, like Asia/Tokyo or America/Argentina/Buenos_Aires.
• The most populous place in the region usually gives the zone its name, spelled in mainstream English, with underscores for spaces.
• Names are kept stable and only change when the usual English spelling changes.^[4] That's why Europe/Kiev became Europe/Kyiv in 2022.^[5] Old names keep working as links, which the explorer above shows.

A few legacy names without an area, like US/Eastern, and the fixed-offset Etc/GMT zones, whose signs are reversed, are kept for compatibility.

A new zone is made only for a region whose clocks have all agreed since 1970. If two places share an offset today but changed their clocks differently in, say, 1985, they get separate IDs so that old timestamps convert correctly. History from before 1970 is kept separately, in a less reliable file called backzone.^[4]

That's why the list of time zones has hundreds of IDs but only a few dozen offsets.

New versions are named by year and letter, and come out every few months, whenever a government changes its clocks.^[7] The latest at the time of writing, 2026e from September 29, 2026, moves Manitoba to permanent UTC−5 from October 31, 2026.^[6]

The project asks governments to announce changes at least a year ahead, so updates can reach devices in time. Changes are proposed on the public mailing list at tz@iana.org.^[7] When the notice is short, people with out-of-date devices can see the wrong time for a while.

The data is built into the GNU C Library on Linux, Android, iOS, macOS, Windows and many databases, and into programming languages including Java, Python and JavaScript. Browsers get it through ICU, the library they use for dates and languages.^[7] Each of those ships its own copy, so updating your operating system and browser is what brings in new rules.

In a browser, Intl.supportedValuesOf("timeZone") lists the zones it knows.^[8] For storing and converting times in code, read programming with time zones.

1. 1.IANA, Time Zone Database. What the database contains, its latest release, and that it's updated when governments change their rules.
2. 2.IETF, RFC 6557: Procedures for Maintaining the Time Zone Database. The "Olson database" name, Arthur David Olson's stewardship, and the TZ Coordinator's role.
3. 3.Computerworld, Time runs out for timezone lawsuit. The 2011 lawsuit, IANA taking over hosting, and the lawsuit being withdrawn in 2012.
4. 4.IANA, Theory and pragmatics of the tz code and data. How zones are named and split, the 1970 rule, and the backzone file.
5. 5.IANA, Time Zone Database release 2022b. Europe/Kiev being renamed Europe/Kyiv.
6. 6.IANA, Time Zone Database release 2026e. Manitoba moving to permanent UTC−5 on October 31, 2026.
7. 7.IANA, Sources for time zone and daylight saving time data (tz-link). How often releases come out, the year's notice asked of governments, the mailing list, and the systems that use the data.
8. 8.MDN, Intl.supportedValuesOf(). Listing the zones a browser knows.