Hours from date to date

I have field where is writing hours, and I have one filed start-date after then other field start-hour. And I have field end-date and end-hour.
How can i calculate how many hours are between start and end. If one day is 8 hours's and  working day is from 8 o'clock until 10 o'clock. And how can i make to not calculate sundat and saturday and hollidays?

It is hard but i need this, thank you