The tz database: the file every device quietly depends on
Why your phone knows Kathmandu is 45 minutes off, that Arizona doesn't change its clocks, and what the rules were in 1987.
The short answer
The IANA time zone database — the “tz database” or “zoneinfo” — is the shared record of every time zone's rules, including their full history.
It is why your phone knows Nepal is UTC+5:45, that Arizona is exempt from daylight saving, and what the rules were on a date decades ago.
What is in it
For every zone, the complete rule set: the standard offset, whether and when daylight saving applies, and every change those rules have ever undergone.
The history is not trivia. Converting a timestamp from 1995 correctly requires knowing the rules as they were in 1995, not as they are now. Financial records, legal documents and archives all depend on this.
Zone identifiers
Zones are named Area/City: America/New_York, Europe/London,
Asia/Kolkata, Australia/Brisbane.
The city is a label for a set of rules, not a statement about geography.
Everyone on US eastern time is America/New_York, whether they live in Miami or
Toronto. The convention picks a large, well-known city in the region — and
deliberately avoids country names, because countries change and split while the
rules attached to a place tend to persist.
Where a country's rules diverge internally, it gets more identifiers.
America/Phoenix exists separately from America/Denver precisely because
Arizona does not observe daylight saving.
How it gets updated
It is maintained collaboratively, coordinated through IANA, and released several times a year — more often when governments make short-notice changes, which they do.
Recent examples: Mexico abolishing nationwide daylight saving in 2022, Iran abolishing it the same year, Chile repeatedly adjusting its dates. Each of those required a database release, and every device in the world was wrong until it picked the update up.
This is the practical reason to keep systems patched. A server with a stale tz database silently produces wrong local times for the affected region.
Why this matters even if you never touch it
It is why your phone changes on its own. Your device knows its zone identifier, looks up the rules for the current date, and applies them.
It is why a calendar invite survives a clock change — if it stored a zone. An invite that stored a fixed offset has stepped outside the database and is frozen at one season.
It is why converting against a city is reliable and converting against an abbreviation is not. The abbreviation is a display label; the identifier is the key to the rules.
The historical oddities it carries
The database records that Dublin ran 25 minutes 21 seconds behind Greenwich until 1916, that Samoa skipped 30 December 2011 entirely when it jumped the date line, and that Kiribati created UTC+14 in 1995.
Some entries reach back to the 19th century, when cities kept their own solar time. Those pre-standardisation offsets are recorded as local mean time, in whole seconds — which is why a very old timestamp can convert to something ending in an odd number of seconds.
Related: Time zone abbreviations decoded · How does daylight saving time work?
Timezone: World Clock Widget
A world clock, converter and live day/night map for iPhone — 325 cities, offline, no account.
Figures on this page are computed from the IANA time zone database. Last generated 2026-08-18.