Calculating late fees every month by date

[Calvin Witcher]

I'm creating a chart for a company to calculate late fees. There is a start date, 5-day grace period before it is late and then and end date (in which the payment was received).

Every month after the 5-day grace period, a late fee of 1.5% is accessed every month until payment is received.

This is what I mean:

John owes: $100 (due on the 1/1/2008)
John pays the $100 on 4/1/2008)
- there is a 5-day grace period that occurred 1/1-1/5 and on 1/6 was technically late.
That makes John 3 months late...he owes 1.5% of 100 per month...

please help - you can reply here or email me at calvinwitcher@yahoo.com