Subtract times and let result go negative

[Mr Wit]

Total hours available less hours worked each day, allow results to go
negative. Excel displays negative times as #.

 

[daddylonglegs]

You can generate negative times if you use the 1904 date system

Tools > Options > Calculation > 1904 date system

....although if you use on an existing worksheet it'll make all your
dates 4 years later!