That's seems like an incredibly silly things to bother with, why not just set each month to have the corresponding number of days with leap years included? Or create months with a maximum number of 31 days? Heck, why even bother with 32? Sure it might argue its a power of 2, but its not like that would actually help with data storage, both could be fit into 5 bits, but even if that's the case, I'm almost certain its a bug because of how unregulated the structure is. Go to the First Astral Moon, the date change is between the 28th and 29th, now go to the Third Astral Moon, the date change is between the 29th and 30th even though both have 31 days. It's incredibly messy and arbitrary. Should they have wanted to provide players with a way to represent their birthday they could have simply let it match the dates and then no one would need to pick the 32 if they wanted to have their real birthday match their character's instead they have a system where the days meaninglessly diverge from their intended dates. For instance If my birthday was on (4/9) it would for some reason have to pick the 10th in game?

It seems like they should either not bother showing that information or fix the numerical divergence, since it just looks sloppy and poorly executed if you look into it.