Excel 2007 : Financial Formula

Hi, need a formula.

I'm trying to calculate a loan payment that i want to auto-adjust each time i pay some of the principle.

For example I have a due date of 10/01/2012. I want it to formulate todays date and the target date and give me the weekly payment due.

Amount owed is $2000
Target Date is 10/01/2012

so there's 52 weeks left on the loan. Next week when i check it, i want the formula to change to 51 weeks, so on...

I can't find the date formula that will do this for me. I was going to use the DAYS360 formula but for whatever reason when i use the DAYS360, it always kicks out "0" as the answer.

Who's got the answer and what is it?

Thanks in advance.