> The most surreal part of implementing the new calendar came in October 1582, when 10 days were dropped from the calendar to bring the vernal equinox from March 11 back to March 21. The church had chosen October to avoid skipping any major Christian festivals.
The "original" Julian calendar was indifferent to year number systems. The Romans typically used the consular year, although Marcus Terentius Varro "introduced" the ab urbe condita (AUC) system in the 1st century BC, which was used until the Middle Ages. From the 5th to the 7th century, the anno Diocletiani (also called anno martyrum) after emperor Diocletian was used primarily in the eastern empire (Alexandria), or the anno mundi (after the creation of the world). It was Dionysius Exiguus in the 6th century, who replaced the anno Diocletiani era with the Anno Domini era. His system become popular in the West, but it took a long time until it also was adopted in the East. Its application to years before the birth of Christ is very late: we come across it first in the 15th century, but it was not widespread before the 17th century.
All these systems used the Julian system for months and days, but differed in terms of the year and (partialy) in the first day of the year.
The century in which the switch occurred (which was different in different countries) was shorter than the others. As were the decade, year, and month in which the switch occurred.
https://www.britannica.com/story/ten-days-that-vanished-the-....
> The most surreal part of implementing the new calendar came in October 1582, when 10 days were dropped from the calendar to bring the vernal equinox from March 11 back to March 21. The church had chosen October to avoid skipping any major Christian festivals.