This is not that insane, or at least relatively insane, because this matches the behavior of localtime(). I have heard the reason for using a 0-indexed month is for the convenience of mapping to an enum of month names.
IMHO a default month of January makes rather little sense, and would tend to facilitate subtle bugs... it seems you'd want the user to specify a month always, or maybe have a default of "indeterminate" or "Nevember"; the first position of an enum is very often the "nothing" case.
