How do I calculate a figure in Excel dynamically based on a known balance and repayment date?

I'm using Excel for Mac 2011. I have a balance in a cell on the spreadsheet (that will change) and a known payoff date (September 4, 2011). I would like to calculate a figure that I have to pay weekly to payoff the balance by the payoff date. I would like it to update dynamically. For instance, if I open up the spreadsheet next week and I haven't made a payment yet, the figure will increase to compensate for having less time to pay off the balance.