IANA time zone notes

Where every offset on this site comes from, how often it changes, and what we do when it does.

The database

The IANA time zone database — the tz database, tzdata — is the shared record of what clocks have done and are expected to do, maintained by volunteers and shipped with most operating systems. It is the reason your laptop knows when Berlin last changed clocks without asking anyone.

Zone identifiers

Zones are named Area/Location after a representative city rather than a country, because borders move and clock rules do not follow them tidily. Aliases are kept as links, so Asia/Calcutta still resolves to Asia/Kolkata. We display the current name and accept the old one in search.

Releases

Releases are named by year and letter — 2026a, 2026b — and appear whenever a government changes its mind, which is a handful of times a year and rarely with much notice. This site is built against release 2026c and rebuilds daily, so a page you loaded last month is not quoting last month’s rules. The build fails outright if the bundled database is older than the newest published release.

What the data does not promise

Offsets before 1970 are recorded but not guaranteed, and future dates are predictions: they assume today’s rules stay in force. When a country announces a change late — it happens most years — a date that was correct last week can be wrong, and every tool that reads this database, including this one, is wrong with it until the next release.

Corrections welcome. If a clock change on this site disagrees with your own government’s announcement, tell us; we would rather patch a zone by hand than wait for the next release.

Attribution

Offsets and daylight saving rules are derived from the IANA time zone database, which is in the public domain. City names, coordinates and populations come from GeoNames, used under CC BY 4.0. Sun times are computed separately and are accurate to about a minute. Full provenance is on the Method page.