What Is the IANA Time Zone Database?
Last updated September 25, 2026
One-line definition: The IANA Time Zone Database (tz database) is the dataset that tells your phone, browser, and server exactly which UTC offset applies to a named location on a given date, including every daylight saving rule.
Why “EST” Isn't Enough for Software
An abbreviation like EST is a label, not a rule. It doesn't say whether daylight saving applies this week, or what happened to that offset in 1970. Software needs something more precise, so it looks up a zone name instead: America/New_York.
That name is a key into the tz database, which already has every rule stored: New York runs EST (UTC-5) in winter and EDT (UTC-4) in summer, on specific dates that shift slightly year to year. Look up Asia/Kolkata instead, and the database returns a fixed UTC+5:30 with no DST rule at all, because India never switches.
Where This Database Actually Lives
It ships inside almost every operating system and language runtime you use. Linux and macOS keep it under /usr/share/zoneinfo. Python's zoneinfomodule reads from it. Java's java.time, JavaScript's Intl.DateTimeFormat, and most databases all pull the same underlying data.
That's the point: one shared source of truth, so a meeting time computed on a Linux server and displayed in a browser on someone's phone agree with each other.
Who Keeps It Updated
A small group of volunteers maintains it through a public mailing list, published under the IANA (Internet Assigned Numbers Authority) since 2011. It goes back further than that: Arthur David Olson started the project in the 1980s, which is why old references still call it the Olson database.
It updates several times a year. When a country moves a DST date, changes its UTC offset, or splits a time zone, that change lands here first, and then flows out to every device the next time it updates its system files.
Frequently Asked Questions
What is the IANA Time Zone Database?
A public-domain dataset, also called the tz database or zoneinfo database, that records the UTC offset and DST history for every named time zone in the world, such as Europe/London or Asia/Tokyo.
Why do zone names use a City, not a country?
Because time zone rules are set at a more local level than most countries realize. The US alone needs America/New_York, America/Chicago, America/Denver, and more, since each observes a different offset.
Is Asia/Kolkata the same as IST?
Yes. Asia/Kolkata is the tz database name for India's single time zone, which everyone knows as IST (UTC+5:30). The database name is what software uses internally; IST is what people say out loud.
What happens if my software's tz database is outdated?
It will apply the wrong DST dates or offset for any zone that changed rules after the last update, which can throw calculated times off by an hour. This is why operating systems push tz database updates automatically.
Related Tools & Terms
Source: IANA Time Zone Database