I wonder how Paradox handles this stuff in their games like Europa Universalis. Have they ever made a "the pope wants you to switch calendar systems" event which changes the actual in-game date?
“My Lord, you can attack them until March of next year!”
“Then March of next year it shall be!”
Congress did something similar last year - National Emergencies declared by the president, such as for tariffs, are required by law to be reviewed within 15 days by Congress. So Congress redefined the calendar so that the remainder of the days during that session are not officially considered to be calendar days. https://www.nytimes.com/2025/04/10/us/congress-johnson-calen...
Checking the wiki, eu5 has an advance (guess these are like the nation ideas in eu4?) for Julian calendar which gives you +10% to orthodox or miaphysite nations. I doubt it has any effect in the calendar system in the games UI.
Paradox's Clausewitz game engine seems to handle "negative" years very poorly, so stuff like historical Roman emperors in Crusader Kings have some oddities. It's probably also why Imperator: Rome uses ab urbe condita dating (aside from immersion).
The Elder Kings 2 mod actually has custom date handling (it's actually a custom date localisation system) to enable transitioning to the 3rd era on founding the Empire of Tamriel.