Sources and methodology

This page records how dates on Online Calendar are produced and where they come from.

UK bank holidays

Primary source: GOV.UK bank holidays and the JSON feed GOV.UK publishes for that page. We store holidays per nation (England and Wales, Scotland, Northern Ireland) with the source URL and a last-verified date. Substitute weekdays are included when Christmas Day or Boxing Day falls on a weekend, as listed in that feed.

GOV.UK is the authority. If our copy and GOV.UK disagree, use GOV.UK.

Gregorian calendar

Month lengths and leap years follow the Gregorian rule: a leap year if divisible by 4, except centuries, unless divisible by 400. So 1900 is not a leap year and 2000 is. Supported display years are 1900–2100. Only years marked as published in administration are offered to search engines.

ISO week numbers

Week numbers follow ISO 8601 via PHP’s DateTime. Week 1 is the week that contains 4 January (the week with the year’s first Thursday). The week starts on Monday even if you print a Sunday-start grid.

Chinese lunisolar calendar

Civil conversion for 1900–2100 using packed month-length and leap-month data derived from Hong Kong Observatory civil calendars (the widely used “lunarInfo” encoding). Chinese New Year is the first day of the first lunar month in that dataset. Zodiac animals follow the standard 12-year cycle. Religious and some regional almanacs can differ by a day.

Islamic civil calendar

Hijri dates use the tabulated “Kuwaiti” arithmetic calendar (common in civil software). Month names are the standard twelve. Moon-sighting calendars can differ. Ramadan and Eid dates shown as “civil” are from this algorithm, not a fatwa or local mosque.

Time zone

The public site uses Europe/London. “Today” on the homepage follows that zone, including British Summer Time.

What we do not claim

We do not publish Tamil, Gujarati, Malayalam, Nepali, Hebrew, Julian or official school-term calendars. Those need source almanacs we do not currently licence.

Related: disclaimer.

Last updated 22 August 2026.