Calculate Add'l Hours on Timesheet per day

I have a timesheet that was created years ago and I want to update it to calculate the additional hours rather than manual entry, yet I have never worked with time calculations.

Each person inputs their In & Out time manually. I would like the Add'l Hours cell to calculate anything greater than 7.5 hours (my office works 37.5/wk & 7.5/day) within the Time In & Time Out entries.

The Hours Accumulated cell will total up all the Add'l Hours cells within one week.
The Hours Remaining cell will take the Accumulated Hours and subtract the Hrs Taken within one week.

I can't seem to get this to work. The example I have in the attached spreadsheet is:

Time In: 7:00 (formatted h:mm)
Time Out: 19:00 (formatted h:mm)
Add'l Hours: =(D17-C17)-7.5 (formatted h:mm) -- I get ##### in the cell and don't understand why?

Want the Add'l Hours to calculate as such: =12:00-7:30 = 4.5 or 4:30 or 4 hours 30 mins (not sure which way I want it displayed yet)

Any ideas?