Formula to Calculate: Start Date + Duration (Days) = End Date ; Business Days M-F

Question

Basically, I'm trying to use SharePoint as some what of a project tool for tasks.

On a new task, we have "START DATE", and a custom number field "ETC". I'd like to have a calculated field "END DATE" populated by adding the ETC (estimated time to complete) to the START DATE, but the result should display a date that is within a business work week (Monday through Friday).

i.e.

Start Date = 4/8/2015

ETC = 8 Days

End Date (excluding weekends) = 4/20/2015  (Calculated Field)

Answer 1

=[START DATE]+[ETC] 
+ ROUNDDOWN([ETC]/5,0)*2 
+ IF(WEEKDAY([START DATE])+MOD([ETC],5)>=7,2,0)
- ROUNDDOWN(WEEKDAY([START DATE])/7,0) 
+ IF(AND(MOD([ETC],5)=0,WEEKDAY([START DATE])=1),-2,0)  
+ IF(AND(MOD([ETC],5)=0,WEEKDAY([START DATE])=7),-2,0) 

Here's where I copied this from.