Hi bwilcox,
I don't think you can do this as you describe. The reason it will not work this way is that the Parameter fields are stored as text in the database, no matter the field type. The metadata then translates them to the proper field type as they are pulled from the DB.
It could be there's a way, but it probably involved wrapping your search's call to the parameter field with a function that converts from text to date/time. Have you tried anything like that?
Jon