Formula for counting down to a due date

i have a date that i supplied goods to a customer e.g 17/7/2013 in cell A1 (date supplied) and the total amount of the goods supplied in cell B1, which the customer has 30 days for the payment to expire, so i need a formula that will countdown each day(i.e from 30, 29, 27 ---------0 and it will bring out the amount as well (i.e 4 days remaining to paid up the value which is 2000). also i have a table in excel which goes like this: Name of Customer/ Amount Paid/ payment due date /0-5days/6-20days/21-40day. so when the decrease fall on 6-20 days, the payment reminder should move to 6-20 day. When it reaches the deadline day, which is 30 days the reminder should move to payment due.

Note: the countdown should always be automatic update without me opening my system always.
Here is my attached format in excel sheet.