How to add month

Hello,

How can we achieve functionality of adding months like if we two fields are of type date and we want to add them up.

Eg: var month1 = this.date.getmonth();

       var month2 = this.date2.getmonth();

so if month1 = 1 , month 2= 3;

total month = month1 + month2; i.e. 4 not 13... as this feild becomes date type i am getting 13 as a result.

If anyone can provide inputs on this.

Thanks and Regards,

Stuti