I need help!

I organize an anual house boat trip which has grown to be quite large. I am looking to update my spread sheet and make it a little more automated. I need a way to calculate the guests total amount due based on the following information:

1. a 7 day stay = $85 per day charge if paid in full by to 01/01/01
a 7 day stay = $100 per day charge if paid on or after 01/02/01

2. a 6 day or less stay= $100 per day charge if paid in full by to 01/01/01
a 6 day or less stay= $115 per day charge if paid on or after 01/02/01

My old spread sheet works well but it cant calculate the payment discount / penalty as it applies to the pay date.

The amount per day charge is currently in a cell on the spread sheet. Once i deterine that amount for the year i can lock the sheet. Then the only user input on the sheet should be the guest name, the pay date, and the number of days stayed.

I tried to figure this out but its over my head. I would really appreciate any advise or formulas you could offer up. Thanks in advance!!!

Brian