Hey gmo, how are ya? Funny bumping into you in this thread...
For historical schedules, the proper day/date combination is important, since otherwise it throws off what the schedule should be. For fictional schedules, it is less of an issue. It really depends on how much a correct real-world calendar matters to the player.
There was posted somewhere in this forum a list of what "Year starts on" values to use in OOTP to get the calendar to match it's real-world counterpart, from 1870 to 2080, if I remember right. That's only necessary because OOTP doesn't realize leap years exist (gee, even the Romans knew about leap years!

)
Hopefully OOTP7 will get an overhaul in the calendar department and it will feature the true calendar for any year one chooses. Incidentally, PureSim does use the true calendar in its date keeping.