At the moment you are only able to set up repeat billing for a date in the future that also sends invoices every "So many" days. E.g. Repeat billing to start on the 01/11/2015 sending invoices every 30 days.

This has no benefit to those of us who invoice clients on the same day every month e.g. the 30th of every month.

I suggest that as well as being able to set repeat billing up to start on a date in the future, you are also able bill on the same day of every month and use both of these functions at the same time.