This won't work. Such
incomplete or
uncertain dates don't play well with metadata standards, date and time formatting, File Windows, the Timeline, Events, sorting, searching, filtering etc.
Not worth the trouble, really.
The typical way to handle this is to use dates like
YEAR:01:01 if you really don't know more than the year.
YEAR:06:01 If you want to record "first half" of YEAR
YEAR:07:01 If you want to record "second half" of YEAR
Then use a metadata field, category, collection or Attribute to record that the file has an
uncertain date.
This allows you to use standard dates which play well with all related features in IMatch and IMatch Anywhere and other software. And you still know which files have only uncertain dates.
See
Working with Uncertain Dates for more info.