Subtracting 8 hours from a time worked field.

I have a formula field given the result of Date/Time Start and Date/Time end which works fine.  For example such a result would be 9:00 (1 hour overtime given the day is 8 working hours).

My field is 'Overtime for the day' and set as a formula.  How do I define the formula so that I get 1 hour?

Currently I get 32399992.00

My formula is:  'Hours Worked' - 8 (Hours Worked shows 9 hours)