How to utilize date add function in Google spreadsheet?

Question

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

Answer 1

You can use the DATE(Year;Month;Day) to make operations on date:

Examples:

=DATE(2013;3;8 + 30) give the result...  7 april 2013 !
=DATE(2013;3 + 15; 8) give the result... 8 june 2014 !

It's very surprising but it works...