Data sources
The datasets, standards and official sources behind every answer, with the version we use and its licence.
Datasets
The data every computed answer is built from. We ship our own copies so answers do not depend on the version built into your browser.
| Source | Version | Licence | Used for |
|---|---|---|---|
| IANA Time Zone Database (tzdata) | Release 2026f, published 2026-10-27 | Public domain | Every UTC offset, abbreviation and clock change; zone IDs and aliases. |
| Unicode CLDR | Version 49 | Unicode License v3 | Time zone names (such as “Japan Standard Time”), the meanings of abbreviations, the first day of the week and the weekend by country. |
| GeoNames | cities5000, countryInfo and admin1 codes, retrieved 2026-11-02 | CC BY 4.0 | City names, countries, regions, population, coordinates and calling codes. We use a subset of 25,000 places and a few of their fields, and assign our own URL names. |
| ICU Umm al-Qura data | Month-length table 1300–1600 AH | Unicode License v3 | The Islamic (Hijri) date in the Umm al-Qura calendar. |
| IERS leap-seconds.list and Bulletin C | Bulletin C 72, 6 July 2026 | Public domain (leap-seconds.list) | The 27 leap seconds since 1972, TAI − UTC = 37 s, and the announcement that no leap second is due at the end of 2026. |
Standards
The definitions behind formats, week numbers and day counts. Standards that are not freely published are cited, not reproduced; where a public profile restates them, we cite that too.
| Source | Version | Licence | Used for |
|---|---|---|---|
| ISO 8601-1:2019 and ISO 8601-2:2019, with amendments | 2019; Amd 1:2022 and Amd 1:2025 | Standard (cited, not reproduced) | Calendar dates, week dates, ordinal dates, durations and UTC offsets. |
| RFC 3339 and RFC 9557 | 2002; 2024 | IETF Trust | The internet date-time profile of ISO 8601, and time zone names in brackets. |
| RFC 5322 §3.3 and RFC 9110 §5.6.7 | 2008; 2022 | IETF Trust | Email and HTTP date formats. |
| RFC 5905 (NTPv4) | 2010 | IETF Trust | The offset and round-trip formulas behind the clock check. |
| POSIX (IEEE Std 1003.1-2024), “Seconds Since the Epoch” | Issue 8 | Cited | Unix time and the 86,400-second day. |
| WHATWG HTML: week strings | Living Standard | CC BY 4.0 | The ISO week rule restated: week 1 contains the first Thursday. |
| IS-GPS-200N | Revision N | US government work | The GPS time scale, epoch and week number. |
| Microsoft: Excel date systems and the 1900 leap-year bug | Consulted 2026-11-02 | Cited | Excel’s 1900 and 1904 date systems and why 1900 counts as a leap year there. |
| Discord developer documentation: message formatting | Consulted 2026-11-02 | Cited | Timestamp markup and its styles. |
Time services and law
Official sources for UTC, leap seconds and the legal rules of clock changes.
| Source | Version | Licence | Used for |
|---|---|---|---|
| BIPM: CGPM 2026, Resolution 5 | October 2026 | BIPM | The decision that UTC becomes continuous, without leap seconds, from 20 May 2027. |
| NIST: time.gov, Internet Time Service, time FAQs | Consulted 2026-11-02 | US government work | UTC(NIST), network-delay correction, 12 a.m./12 p.m. ambiguity, UTC and GMT, US daylight saving time. |
| gps.gov: GPS and telling time | Consulted 2026-11-02 | US government work | The precision of GPS time. |
| 15 U.S.C. §260a | US Code 2023 | US government work | US daylight saving time: second Sunday of March to first Sunday of November. |
| Directive 2000/84/EC | 19 January 2001 | EU | Summer time in the EU: 01:00 GMT on the last Sundays of March and October. |
| Summer Time Act 1972 and Interpretation Act 1978 s.9 | As amended (SI 2002/262) | Open Government Licence v3.0 | British Summer Time and GMT as UK legal time. |
Calendars
Sources for the calendar rules and their history.
| Source | Version | Licence | Used for |
|---|---|---|---|
| US Naval Observatory: leap years, calendars, Julian Date | Consulted 2026-11-02 | US government work | The Gregorian leap year rule, mean year lengths, the October 1582 switch, and the Julian Day algorithm (Fliegel and Van Flandern) with check values. |
| US Social Security Administration, POMS GN 00307.180 | Consulted 2026-11-02 | US government work | The dates on which countries adopted the Gregorian calendar. |
| Calendar (New Style) Act 1750 | 24 Geo. 2 c. 23 | Open Government Licence v3.0 | Britain’s switch: September 2, 1752 followed by September 14, and the year starting on January 1. |
| e-Gov Japan: Era Name Act and Cabinet Orders | Act No. 43 of 1979; Orders of 1989 and 2019 | Government text | Start dates of the Heisei and Reiwa eras. |
How we use sources
Every rule stated on this site comes from one of the sources above, and the line at the end of each page names the rule that page relies on. Numbers that follow from a rule, such as the years with 53 ISO weeks or the weekday of a future February 29, are computed by our own code from that rule. They are then tested against an independent implementation, usually Python’s standard library, before they are published. We do not publish a claim we could not trace to a primary or official source. Where sources disagree or are silent, we say so instead of picking one quietly.
Attribution
City data © GeoNames (geonames.org), licensed under Creative Commons Attribution 4.0; we have reduced it to the fields we use and added our own URL names. Time zone names, abbreviations and week data © Unicode, Inc., under the Unicode License v3. The time zone database and the IERS leap-second list are in the public domain. UK legislation is quoted under the Open Government Licence v3.0.
What the sources do not settle
Time zone abbreviations are not standardised: the same three letters can stand for zones on different continents, and neither tzdata nor CLDR settles which one a reader means. Pages that use an abbreviation say which meaning they use and list the others. Religious dates in the Islamic calendar are set by moon sighting in many countries and can differ by a day from the Umm al-Qura calendar shown here. The Umm al-Qura dates themselves are computed from the ICU table, because the official tables could not be retrieved for checking. The first day of the week and the weekend are conventions recorded by CLDR, not laws, and countries change them from time to time.
How we keep time explains how these sources are turned into the answers on each page.