Calendar month invoicing

We sell some licence products which are due in full each calendar month.

How can I setup invoicing so it automatically is generated at the start of each calendar month regardless of when the product is purchased?

i.e. a product purchased on March 25th worth $20/month would cost $20 in March, then again $20 in April.

the default appears to be an invoice on the 25th of each month.

Thanks.