Comment by cyost

Some notes:

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.