If you don't have the option of creating a view or SP for whatever reason, I think you could also bring in the second database in a subreport, and create a formula within the subreport that converts the numeric date field to a date and then link the subreport to the first database on this formula. Of course, this would slow the report and make it more difficult to work with data between the tables, since you'd have to use shared variables for calculations, etc.
-LB