User blog comment:LamsunAT/all character's Birthday/@comment-34857175-20181206071233/@comment-26474758-20181208073409

I would guess that this information came from Kumomadori Radio, since a lot of little tidbits of background information seem to be come from there (e.g. the details about Oomama). These two exchanges on Twitter suggest that Arawi Keiichi sometimes lets fans arbitrarily decide little details like this, and then "canonizes" it in the next radio session, so to speak. My Japanese is not great though, so I could be wrong.