Invoice Date on Service Contract

Team... we are doing some testing and are submitting billing transactions (manually) to Receivables.

We need to have the Invoice Date to be the last day of the previous month (Invoice is in Arrears), so for the activity in March we run the invoice on May and we need the Invoice date to be the last day of April. Now, the real trick is as follows... the interface is NOT consistently run on a specific day of the following month, it can vary to be the first up to the 8th day or maybe more. The need is to, no mater when the interface is run, we need the invoice to be dated as