Hi!
I'm hoping you can point me in the direction for these issues, its a shame http://drupalecommerce.org/ is down :(

I have a site that has membership setup as follows:

on 15th september all members 'paid' statuses are set to unpaid. (I have no idea how I'll do this but its not the most important)

15th september users can renew for $100
16th-31st september users can renew for $125
jan 1st users can renew for $150

also if a user is banned and wants to rejoin they pay $175 but I'm thinking that would be a separate product!?
(the dates may change year on year!)

also, if a user joins halfway through the year the fee has to be calculated pro rata.

My questions:
how do I calculate fee pro rata?
How do I make paid status unpaid on x date?

any directions would be really helpful! I have no idea where to start!