Rata Die Converter

Rata Die counts elapsed days from 1 January of year 1 in the Gregorian calendar extended backwards. The name is Latin for fixed date, and the idea comes from Calendrical Calculations, by Nachum Dershowitz and Edward Reingold, which uses the count as the intermediate representation for every calendar it describes.

The reason is practical and applies to any system handling several calendars. Converting directly between pairs needs one function per pair: ten calendars would mean ninety functions. Route everything through a day count and you need two per calendar — one in and one out, twenty in total. Every decent calendar converter is built this way underneath, even when it does not advertise the fact.

Both directions live on the same page, which lets you use the number as a bridge: convert a date to Rata Die here and feed the same number into another converter. Worth noting that dates before 1582 are proleptic — they apply the Gregorian rule to years in which it did not yet apply, which is exactly what a continuous count should do, and not the same as the historical date recorded at the time.