Currently we can only raise a monthly invoice when a month is completed. For example we can only bill for August in September.
Our billing model is (based on August)
1) Raise the monthly invoice on 1st August for the month of August
2) the customer has 30 day terms so pays the invoice by the end of the month
3) On the 1st of September raise the invoice for September and ensure August has been paid for.
We need the option to be able to bill a months retainer at the start of the month, or any date that we choose.