Be able to change the date for future recurring invoices if the first invoice was NOT sent on the date that you want them to recur on.

i.e. send first one today (set up today) on 19th Dec
However, want all future ones to be sent on 15th of month.

Can't do this without changing the date of the start date after the first invoice has been sent. This is not correct to do as accountancy rules mean you can't change dates of invoices that have been sent.