DB design question for Report generation

[heat23]

I have a database of affiliates and I pay them a certain amount of money for each client they bring me. However, the amount I pay can vary from month to month for each affiliate. The problem is when I generate historical reports for each affiliate. I don't want to use the current amount being paid, I need to use the amount that was paid during that particular month. So my question is, how can I design the DB in such a way so that I can accomplish this??

Also, if I want to generate a report for lets say, January 2004, how do I convert that to a timestamp and get all the data for that month if I have timestamps stored in the database?

Thanks for the help!

[Pig]

I might be missing something, but why not have a table with affiliate ids, the amount payed, and the date they were paid. Then you can just do a search like:
SELECT sum(table_amount_paid) FROM table_name WHERE table_associate=$this_associate AND table_date > $start_date AND table_date < $end date