How To Exclude Saturday And Sunday In Date Calculation

I have three columns named "name","start_date" and "end_date"

ex:

Name start_date end_date
AB 9/11/13 10:19 AM ??
CD 9/12/13 11:45 AM ??

All i need to find is,

If Name = AB, i want put start_date+3 days in the end_date column,
similarly for name = CD , start_date+7 days.

But the calculation should exculde weekends(sat and sun).

ex: For AB, start_date = 10/3/13 (3rd oct , thrusday), end_date should be start_date+3 ,so end_date = 10/6/13 but 6th october is sunday so the formula should give end_date as 10/7/13.