Concept
Version 4
Created by Boundless
Dates and Calendars
The first page of the papal bull "Inter Gravissimas" by which Pope Gregory XIII introduced his calendar.
During the period between 1582, when the first countries adopted the Gregorian calendar, and 1923, when the last European country adopted it, it was often necessary to indicate the date of some event in both the Julian calendar and in the Gregorian calendar. Even before 1582, the year sometimes had to be double dated because of the different beginnings of the year in various countries.
Source
Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:
"Inter-grav.jpg."
https://en.wikipedia.org/wiki/Gregorian_calendar#/media/File:Inter-grav.jpg
Wikipedia
Public domain.