Due date reminder in excel 07

Hello,
dear all. I have a "credit list" worksheet having the details like. 1) date 2) invoice number 3) name of beneficiary 4) total amount 5)paid cash 4)discount and 5) net pay in a row ( side by side columns)
I will enter these details on a daily basis one below the other on datewise.
when I raise an invoice , I will enter all these info for quick view of the status of credit given to clients. In principle, we will give credit for 15 days. I mean they must repay their outstanding within 15 days.
my question is : if a particular invoice due date has reached 13 days , I must get some alert ,something like colour change or whatever it may be in a new sheet and also these rows must be recorded(copied) in that new sheet so that I can contact the guys for collection arrangement. how to do this?
one thing , a beneficiery might have several invoices. so I must get alerted of the invoices whose due date is nearing.
pls help me. thanks suryam