In the IPTC spec, it mentions that dates don't have to be complete. E.g. you may only know the year or the year and the month of an image.
So far in my trial of Photo Mechanic, I don't seem to be able to enter a partial date.
Example: I have many slides that are stamped with things like "Jan 75" So I want to enter a date as 1975-Jan WITHOUT a day on it. (I could make do with the convention that it was 1975-Jan-0)
This is common in working with archives.
Similarly you need a way to say "unknown" for time.
If I can't find a workaround, this one is a show stopper for me.