It's a possible addition, but I don't have sense of the demand for it.

I wonder, what does the class offer that a function like this doesn't?

    def adate(date_string):
        return datetime.datetime.strptime(date_string,'%Y-%m-%d').date()

I'd be hesitant to add FileType if wasn't already present.  It hasn't aged very well.  We've had bug issues related to v3 binary files, and contexts.   The main feature that FileType (beyond verifying that the file actually does exist) adds is the recognition of '-' as stdin/out.  

By analogy I'm lukewarm about adding a DateType class.