The "date" format is defined in the XML Implementation Guide that is posted on the NAACCR website; here is what those specifications say about that format:
"A NAACCR-style full or partial date (yyyy, yyyymm or yyyymmdd)."
And here is the regular expression the specifications define for the type:
And so the following are valid dates (no trailing spaces):
The format doesn't allow a known day and unknown year/month or a known day/month and an unknown year.
Many invalid dates (future dates, or dates with a day that is too high for a given month) will be deemed "valid" by this definition; I think the idea was to be able to use a simple regular expression to define what is acceptable and not acceptable and to let edits deal with corner cases.