subtracting one hour and a half

I have a simple MS Excel file, showing the beggining of the day and the end of a day

I have to sum how many hours have elapsed MINUS one our and a half, for lunch time, how can i remove 1   1/2 hour?

Of course i can create 2 aditional columns and count morning and afternoon separatedly, but i think there is a more simple way

AB C

BEGEND TOTAL

08:3219:17B2-A2

Anotther issue,  is to create a 4th colum to sum/count how many hours after 08:00 hours of work has been done by the employee