Project Billing - Invoice format

 Hi

We are doing an implementationin R12.1.3 for, among others,  Project Costing, Billing and OTL . We are now discussion Invoice formats and our customer would like to have the expenditure item dates for labor cost (time reported in OTL) on the Invoice they send away. I can not  find that in the Invoice format form? Is there an easy solution for it ? In this case the billing method is "work".
We are not printing the invoice in PA. We are transfering it to AR and from AR to a third party system that is printing and sending the Invoice to the final destination.