Calculate billable hours between two dates

I am needing to Calculate the billable hours between 2 dates, excluding weekends. 


For example,   10/1  8am    to  10/2  8 am should be just 8 hours. but clearly 24 actual hours.
                        10/1  8am    to  10/2  5pm should be 16 hours, not 33 or whatever.

I have tried every way I can think of to solve this but I cant figure out a good method. Please help. Thanks