When you use a shared or global variable, it must be declared as the same data type in all places. 

I do not understand why you have declared as a number in one place and a string in another place, but you say it is a date.  FYI: For most databases, a date is really a number internally.

First off, so that you can get the value to show in the sub-report.  Either remove the shared variable from the main report or created  a new shared variable (for  testing purposes) that is a datevar datatype and see if you can assign the database field to that variable.  If that works, then change the datatype of the variable to the datevar.  Otherwise, keep trying other datatypes until you can get it work (I do not know why it would not work as datevar, but then I do not now your database).