Dealing with FLAC's carrying Vorbis comments, if that makes a difference. When attempting to save a DATE tag where 00 is used in place of the day/month/both, Picard will strip it from the date completely. For example, "2005-12-00" becomes "2005-12" and "2005-00-00" becomes "2005". If DATE is saved as "0000-00-00" the tag is emptied. Interestingly, "2005-00-12" also becomes "2005-12", and "0000-00-12" becomes "0012". None of this behavior can be overridden using scripts.

This is frustrating because there are releases for which part of or the whole date is unknowable, and using "00" in their DATE tags is a helpful way to represent this phenomenon. As it stands I'm forced to use external/third-party tagging solutions to tag DATE the way I want to which is a bummer.