I believe the issue I am having now should be much easier in MS Excel. However, since my company uses Google Spreadsheet so I have to figure out a way.
Basically, I have
The direct use of EDATE(Start_date, months) do the job of ADDDate. Example:
EDATE(Start_date, months)
Consider A1 = 20/08/2012 and A2 = 3
A1 = 20/08/2012
A2 = 3
=edate(A1; A2)
Would calculate 20/11/2012
PS: dd/mm/yyyy format in my example
dd/mm/yyyy