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
I like to keep it simple. If A1 holds the date and B1 holds the number of months to add, then
=date(year(A1),month(A1)+B1,day(A1))
would calculate the required result. The same way could be used for days or years