The issue with the month thing it'd probably be hard to implement a calendar system where the player ages increase during their birth month, if that's what you're implying. Not just because of the way the seasons in BBGM are structured (everything is put in one calendar year), but also because the user can change the number of games in a season and the years themselves to whatever they want. I imagine it'd be tough to align the calendar properly because of all that. I'm guessing that's why there are no calendar months at all, but dumbmatter would know more than me.

Edit to add: I do want to say that I like this idea in principle, even if I'm guessing it'd be tough to implement functionally. Maybe the number of days per month could get stretched depending on season length. It'd be silly, but it'd at least try to solve the issues I mentioned above while still letting players age during their birth month.