I bill one of my clients in this fashion. He's my previous employer and I agreed to check over his DB system each work day morning to make sure that it's in good shape. It only takes a few minutes each day so I only bill every few weeks.
I set up this relationship:
Invoice::DateStart > LineItems::Date And
Invoice::DateEnd < LineITems::Date
I only have one client on this plan, but if I had more, I'd add ClientID match fields as well. I create my line item records on a layout based on that table instead of via a portal. I can thus create a new record in Invoice, select Dates in DateStart and DateEnd. A portal shows the matching records so that I can confirm that I am getting the correct records. I then click a button that pulls up the lineItems in a list view with fields from Invoices added to complete the invoice, Save a PDF and email it off to him.