Data sources and methodology
Last updated
Every time on TimeThen is calculated from published, authoritative rules. Nothing is typed in by hand.
Sources
| Data | Source | Licence |
|---|---|---|
| Time-zone rules, offsets, DST | IANA Time Zone Database (release 2026e in the current build) | Public domain |
| City list and coordinates | Curated by TimeThen, coordinates rounded to 0.01°. Expanding to GeoNames as coverage grows. | GeoNames: CC BY 4.0 |
| Country codes | ISO 3166-1 | Reference standard |
| Time-zone names | Unicode CLDR via your browser | Unicode licence |
How a time is calculated
-
Each city is assigned one canonical IANA time zone, for example
America/Chicago. - Live clocks are computed on your device from the current instant and that zone's rules. We never call a server every second, and we never store a fixed "UTC-6" for a city, because offsets change with daylight saving time.
- Page facts such as the current offset, the next clock change and time differences are computed when the site is built. The site is rebuilt every day, and each page shows its update date.
Daylight saving edge cases
When clocks spring forward, some local times do not exist (2:30 am on the US change day). When they fall back, some local times happen twice. Our converter follows the same rules as modern software (the ECMAScript Temporal standard): a skipped time moves forward by the size of the gap, and a repeated time uses the earlier occurrence unless you choose otherwise.
Updates and corrections
Governments sometimes change time-zone rules at short notice. We track every IANA release and rebuild the site when one is published. If you spot an error, please tell us.