If I read that correctly, you have a field named Date Billed and you want to see only related records in the portal where Date Billed matches a specified date.
If you add a global date field such as YourLayoutTable::gSelectedDate
You can use this portal filter expression:
YourLayoutTable::gSelectedDate = PortalTable::Date Billed
to limit the records shown to those with a specific date in Date Billed.
But to get the portal to update smoothly after selecting/entering a date in the gSelectedDate field you should include the field in your relationship.
You can either use gSelecteDate as an additional match field with the = operator and not use a portal filter or you can include it as a match field using the X operator and then use the portal filter expression to limit the records shown to the specified date.