How we get our dates

Three kinds of source

• Computed from a published rule. Fixed dates, weekday rules ("the second Sunday of May"), Western and Orthodox Easter, and Hindu calendar (panchanga) dates worked out by the WhatDate date engine from astronomical positions. Each computed pack says which rule it uses.
• Copied from a named authority. Lists published by a government, a religious institution or another organiser, transcribed as names and dates only.
• Editorial. A small number of dates put together by WhatDate from the sources named on the pack.

Checks before a date is published

• Credit to the publisher. A published date is a public fact; we never claim it as ours. Every date names the calendar that publishes it, with a link, on its event page, its pack page and in search. We copy names and dates only, never the publisher's wording.
• Two independent transcripts. A list copied from an institution is transcribed twice, separately, and the two are compared. It is published only when they match.
• Tested calculations. The date engine's automated checks compare its results with published almanacs and with US Naval Observatory moon phases.
• Nothing is scraped. No site is fetched automatically; sources are read and recorded by hand.

Why the same festival can have two dates

Lunar and solar calendars depend on where the moon and sun are seen from and on each tradition's rules for when a day begins. Two sources can therefore list the same festival on different civil dates. When they differ, the page shows each date with its source rather than choosing one silently.

Provisional dates

A date marked provisional has not been confirmed yet: for example, the institution that publishes it has not released next year's calendar. It may change. When the official date is published, we check it and remove the mark, or correct the date.

Keeping packs up to date

• Packs computed by the date engine cover the current year and the next one, and from 1 October also the year after.
• Each January we check the institutions we copy from for their new calendars.
• A confirmed error is corrected within 48 hours. The entry keeps its identity, the pack's version goes up by one, and the change is recorded.

Spotted a mistake?

Report a wrong date or see Contact.