EU3 and EU4 at least had no leap years. I assume 1, 2 and 5 are the same. Checking the wiki, eu5 has an advance (guess these are like the nation ideas in eu4?)

EU3 and EU4 at least had no leap years. I assume 1, 2 and 5 are the same. 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.