Rather than editing the selection criteria, try creating a formula with the calculation and then use that. The drawback to this approach is that it will pull all of the data to the workstation and then do the filtering there instead of doing it on the database (if you're working with a Client/Server db that does processing on the server.)
Another way might be to create a SQL expression and see if you can use that in the selection criteria. What's in the expression will be based on your database type. In Oracle it would be:
To_Date('01-'||{gl.balance.period_name})
If that doesn't work, you may have to see about using a command instead of tables.
-Dell