If I follow this, you're saying that your worksheet calculates hours correctly up to this date, up to TODAY() I mean, but now you want it to calculate how many hours are left. I see two possibilities:

1) If your contract has (say) 2000 hours on it, and your worksheet shows in F23 that so far 823 hours have been expended, then the hours remaining until the end of the contract are simply 2000 - F23 = 1177.

2) If you want to calculate the working hours remaining between today and 2013-09-20, then the number of hours worked so far doesn't come into it at all; it's just a matter of figuring out how many working days there are between today and 09-20 and multiplying by 8-or-whatever. As I recall, Excel has some functions that handle working hours and days in an add-on. Let's see....here it is; NETWORKDAYS "returns the number of whole working days between start_date and end_date. Working days exclude weekends and any dates identified in holidays." But this doesn't say anything about having to install an add-on; maybe they've made it standard in the latest versions of Excel. (I only recently migrated from 2003 to 2010.)