For what it's worth, ExifTool (and by extension, PhotoStructure) support 0 for month and day. The problem is that most all other applications won't see this as a valid date.
And I've struggled with how to covert this "fuzzy" date into something that sorts with other assets that _do_ have an exact date. Should they all live on the first of the month? In the middle of the fuzzy date range? Midnight? Noon?
Yes, I like your software and paid for it. But as you note it's not a solution guaranteed to be portable to another photo framework or even Microsoft file tag management.
DC discussed approximate dates at some point. What does 'circa' 1900 mean to searches by date? Is 1892 circa 1900.
And I've struggled with how to covert this "fuzzy" date into something that sorts with other assets that _do_ have an exact date. Should they all live on the first of the month? In the middle of the fuzzy date range? Midnight? Noon?