Select MAX DATE from joined Table
If you want to get just project id and clients (which are both on project table) there is no need to join Billings :

SELECT p.Project_ID, p.Project_Client 
FROM  PROJECT p 
    WHERE EXISTS 
      (
         SELECT NULL FROM BILLINGS b WHERE b.Project_ID = a.Project_ID 
         AND b.Bill_Date &gt; 'your_date'
      )