The Calendars system is a very complex beast that needs some taming. I don't fully understand all it's nuances yet, as I've focused on other things since coming on board recently, but that's changing now.
It's definitely very flexible, and was designed to support all the kooky calendars from all sorts of fantasy and scifi settings. The real problem isn't so much the calendar engine tech (which is solid), but the interface for making managing the complexity understandable and relatively straightforward.
As far as I'm aware, the only real limitation is that your calandar/date/time system must have some kind of indivisible unit analogous to a second. It doesn't have to be named "second", there don't have to be 60 of them in a "minute" (or whatever you rename
that to) but they have to be consistent. You can't have a "minute" consisting of both "seconds" and "otherseconds" which are different time lengths. To be honest, I can't even really comprehend such a system, so if you're trying to do that you're so far down the road to Crazytown that even software can't help.
There may be other limitations as well, but like the "seconds" debacle above, they are just as likely to be practical limitations as they are technical limitations. For instance, while it may be technically possible to have hundreds of calendars running at once in a Realm, at that point we're really stretching the limits of user comprehension, and we may limit the number to a considerably lower level just to make the interface for dealing with it all even a little bit sane.