Rules/Guidelines:

1. https://musicbrainz.org/doc/Artist#Begin_and_end_dates
2. https://musicbrainz.org/doc/Style/Artist

A character with a precise begin date (month and day filled in) is likely to either be a fictional birthday (incorrect usage of MusicBrainz) or incorrect precision (character creation dates are generally unknowable).  In general, character begin dates would only be expected to have year or month precision, not day precision.