What Is the IANA Timezone Database and How Does It Work?

When your phone automatically adjusts for Daylight Saving Time, it is not magic. It is not checking a government website. It is reading from a single, free, open-source database that almost nobody has heard of: the IANA Timezone Database. It is one of the most important pieces of software infrastructure on the internet, and it was built by one guy in the 1980s in his spare time.

What Is It, Exactly?

The IANA Timezone Database — also called the tz database or the Olson database — is a massive record of every time zone rule in history. Not just the current rules. Every rule. Going back to the 1800s. It knows what time it was in Mumbai in 1906. It knows when Germany switched to DST during World War I. It knows about every political change that ever caused a country to shift its clocks. It is basically the complete history of human timekeeping, in a database.

It was originally built by Arthur David Olson at the US National Institutes of Health in the 1980s — which is why it is sometimes called the Olson database. A guy named Paul Eggert at UCLA has been the main maintainer since 1993. The whole thing is public domain. Anyone can use it for free. And virtually every operating system, programming language, and web browser in the world does exactly that.

How Are Time Zones Named in the Database?

Time zones in the database are named in the format 'Area/Location' — like 'America/New_York', 'Europe/London', or 'Asia/Tokyo'. The Area is a continent or ocean. The Location is the biggest city in that time zone. This naming system was a deliberate choice to avoid political problems — if you named time zones after countries, you would have to rename them every time a country changed its name or borders. Cities are more stable.

Some of these zones cover huge areas. 'America/New_York' covers the entire US Eastern Time zone — every state and city that observes Eastern Time. 'Asia/Shanghai' covers all of mainland China, even though China geographically spans five time zones. The database does not care about geography. It cares about what time the government says it is.

What Does the Database Actually Contain?

For every time zone, the database records the complete history of UTC offsets and DST rules from the earliest available records to today. That includes pre-railway local mean times from the 1800s, wartime DST rules from both World Wars, and every change since. There are about 600 time zone identifiers in the database, though many are just aliases pointing to the same underlying rules.

Each entry specifies the standard UTC offset, the DST rules (including exact transition dates and times), and the full history of every previous rule. This is what allows software to correctly calculate the local time at any location for any point in history — not just right now.

It Is Literally Everywhere

The IANA database is embedded in every major operating system — Linux, macOS, Windows, iOS, and Android all include a copy. It is in Python, Java, JavaScript, Ruby, and basically every other programming language. Web browsers use it to power the JavaScript time API, which is how this website shows you the correct time for all 170+ cities without ever making a server request. Your browser already has the answer.

How Does It Get Updated?

Whenever a government announces a change to its time zone rules — which can happen with very little notice, sometimes just days before the change takes effect — the database maintainers update it. They monitor government announcements, news sources, and a dedicated mailing list. Once a change is confirmed, a new version is released and operating systems push updates to users. The database is versioned by year and letter — '2025c' means the third release of 2025. It is hosted at iana.org and the source code is on GitHub. Very transparent.

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