Method

Every number on this site comes from somewhere. This page says where, and what we do to it.

Offsets and daylight saving rules

From the IANA time zone database, which is in the public domain. We read it through the platform’s own machinery rather than bundling a second implementation, so there is only ever one answer in play — and the build hands that machinery the current release rather than waiting for the runtime to ship it. Release 2026c is what this build was compiled against; the IANA notes page covers cadence and what the data does not promise.

Clock-change dates are not transcribed from anywhere. Each one is found by searching the database for the instant a zone’s offset changes, so the daylight saving page cannot drift out of date relative to the rules the rest of the site is using.

Cities, coordinates and populations

From GeoNames, used under the Creative Commons Attribution 4.0 licence. We include every settlement over 100,000 people, which is 4,449 cities, and hand-curate 218 of them as the list the planner and the directory show by default.

Each city’s time zone is resolved from its coordinates against the tz boundary data rather than from its country, because several countries span more than one zone and a few zones span more than one country.

Sunrise, sunset and day length

Computed from the city’s latitude and longitude with a standard solar position algorithm, accurate to about a minute. Nothing is quoted from a third party, and polar days with no sunrise or sunset say so rather than printing a plausible-looking time.

What we do not do

We do not average offsets, round half-hour zones to whole hours, or use an abbreviation as an identifier. Where a zone has no accepted abbreviation, we print the numeric offset, because that is what the database itself publishes. Where a local time does not exist — the hour skipped on a spring-forward morning — we say so instead of silently picking a neighbour.

Corrections

If something here disagrees with your own government’s announcement, please tell us. See Contact.