I think that the best approach to your problem is to use a date island as the dimension. This is a date table that is not associated with the document data. I use a prefix is. on the field name of the island fields (is.Date, is.Month etc). Then you would use statements along the line of:
Documents Added expression:
=Count(Distinct If(DateAdded = is.Date, DocID))
Documents Reviewed expression:
=Count(Distinct If(DateReviewed = is.Date, DocID))
i know Sum(If()) type expressions do not always perform well, but this should not be a problem unless your DocID table contains many millions of rows. And you cannot use set expressions here.
Hope that helps