Lookup function to take into account working days only
 

Hi I have set up a lookup formula to calculate the projected finish time for
a job in the format dd/mm/yyyy hh:mm , this is:

=LOOKUP(J38,{1,2,3,4,5},E38+{"0.083333333333333333 333333333333333","0.166666666666666666666666666666 67","1","2","7"})

E38 is the date/time that the job was logged
J38 is the prority status (1-5)

The priority status related to:
1-completion should be within 2 working hours
2-" " " " " 4 working hours
3-" " " " " 1 day
4-" " " " " 2 days
5-contact should be within 2 days

How can i add to the formula using =if to make the formula not give days
which are weekends or out of hours (after 5pm to 8.59am)

Thanks.