Methodology and sources

Time zones and clock changes

Every time, every offset and every clock change date on the site is worked out from the IANA time zone database (tz database). Maintained by volunteers under the Internet Assigned Numbers Authority, it records the history and rules of legal time in every region of the world since 1970, and often well before. It is the database used by Android, iOS, macOS, Linux and most software.

Every day, the server compares its copy of the database with the latest version published by IANA and warns us if it is out of date; version currently in use: 2026f. Governments’ decisions (dropping summer time, changing time zone) are published with their sources in the release notes.

Browsers have their own copy of this database, often updated several weeks late. The site’s clocks do not depend on it: the server sends, with each page, the calendar of offsets for the time zones shown, for the months ahead.

The exact time

The server is synchronised with reference time servers using the NTP protocol. When a page opens, your browser queries it three times, measures each round trip and keeps the fastest: the offset between your device and the server is estimated to within half that round trip, a few hundredths of a second on a good connection. The time shown is corrected for this offset, and the margin is shown below the clock. Details and setting your clock

Cities

Cities, their coordinates, population and time zone come from GeoNames, under the Creative Commons Attribution 4.0 licence. English names are GeoNames’ English names, excluding historic and colloquial names. We make corrections: neighbourhoods recorded as cities are merged into their city, a city filed under a neighbouring country’s time zone is brought back to its own country’s zone, and when several cities share a name, the short address goes to the most populous one.

The sun and the Moon

Sunrise and sunset are worked out from each city’s coordinates, for a clear horizon, when the upper edge of the sun appears or disappears, allowing for average atmospheric refraction. Civil dawn and dusk correspond to the sun 6° below the horizon. Terrain, altitude and the day’s refraction can shift these times by a few minutes. Moon phases, moonrise and moonset, seasons and eclipses follow Jean Meeus’s astronomical algorithms and NASA’s eclipse data; they have been checked against the US Naval Observatory.

Limits

• Future dates follow the rules known today: a government can change them at short notice, and the IANA database is updated within days.
• For cities close to a time zone border, the time used is that of the city’s time zone according to GeoNames and the IANA database.

Reporting a mistake

A time, a name or a position that looks wrong: write to us, with the page address.