Partial Release Date of Books

When I manually add a book not present on Amazon, Listal asks for the release date. Problem is, many books only show print year, or year and month, not the day. For instance: May 2000. Listal seems to accept a partial release date like "May 2000" (no day), so users think everything is okay, but then "May 2000" is not stored into Listal database. Instead, the system uses the default date December 31, 1969!
Same goes if you try to edit a Release date but only enter the year or month a year, while leaving blank (actually a dash "-") the day field. Result is: "Nothing has been changed".