1. This column formula should give the number of working days between the two days:
Try(Sum(Day Of Week(Index(:A,:E,86400))<6)-1)
Paste this into the Formula editor. The Try() is only needed if you have empty cells in the date column. The -1 is there to exclude the start (or end) date from the sum. 86400 is the number of seconds in 24 hours.
2. The column formula equivalent to PMroz suggestion would be:
How about using the Column Info and specify Start Date and End Date columns with Data Type = Numeric, Modeling Type = Continuous, Format=Date=m/d/y, Input Format=Date=m/d/y. Then add an additional column and using the Column Formula specify End Date-Start Date and also choose Format Duration :day:hr:min
If your start and end date columns are Continuous, Numeric, Date format; then the formula for the duration between the two columns in days could be something like: Date Difference(:Start Date, :End Date, "day")
If your date columns are character, then you can parse the text like PMroz did above with: