How to bill cards x days after invoice date; yet retain rental period as calendar month

[linburnlane]

We invoice all of our customers for a product rental by the calendar month; let's say 1/6/17 - 30/6/17

The invoice date is always the 1st of the month, eg. 1/6/17 - however we only want to bill their cards 7 days AFTER the invoice date.

I've set the Invoice Generation option in Automation to 0.

But if I change the Next Due date to 7/6/17, it then changes the rental period to 7/6/17 - 6/7/17.

I need the rental period to stay as 1/6/17 - 30/6/17 but the billing date to be 7/6/17.

I was hoping it would just be a simple setting I've overlooked in WHMCS but from the research I've done, I believe I need a hook??? apparently, not the fishing kind...at least THAT I understand!