Thank you, Kirk.
I was pretty sure that "no" was going to be the answer before I posted the question. But, I wanted to ask anyway.
From my research, capturing incomplete dates is a real headache for scanning projects. I am considering capturing the date part of the filename (e.g. 197203XX) and adding as a keyword attribute to indicate the "date captured" is an approximation for the benefit of future viewers of the archive.
EDIT: Since I name scanned files in the format YYYYMMDD_Location, I can use PM's ability to extract a substring of the file name to capture the information I want using "date taken: {filenamebase:4,2}/{filenamebase:6,2}/{filenamebase:0,4}". I capture this in the keyword field. It is then easily searchable and provides info to the reader that the date is either an estimate (03/xx/2001) or an actual date (12/25/2003).