That won't quite work.
Performing a find finds records in the parent table not the portal's. If you specify search criteria in the portal fields, you are telling FileMaker to "find all records in the parent table that have at least one portal record with this criteria". Once the find is performed, the portal displays all related records, not just the portal records that match your specified find criteria.
I suggest that you create a report layout based on the portal's table, if possible. Then you can include fields from the parent table on the layout in a header or sub summary layout part as needed to include that data. A find performed on this layout will find the portal records so you avoid the issue that keeps this from working on the original layout.
Another option is (if you have FileMaker 11 or newer ) to specify the date or date range as part of the portal's filter expression and then the portal filters out records not matching that criteria.