Code:

 iff(GNRD(Date_Folder), iff(!GPRD(Date_Folder) OR GPRD(Date_Folder) < [Done] ,GNRD(Date_Folder),GPRD(Date_Folder)) , [Date_Folder])  

 
 
will return the active date of the Date_Folder even if the date is a recurring date.
 
If the date_folder uses unusual characters,  you can put in quotes,  such as:  GNRD("Odd Date Folder Name !()")