How to add month to entered datetime value?

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Javascript

jQuery

I have one textbox in which i am getting one datetime value for example '7/27/2015 12:00:00 AM', and I have one dropdown list control with the list of Monthly,Quarterly,HalfYearly,Yearly. So how to add only months on that date by changing the dropdown periods?

I tried like below but getting extra months and year not changing if month get above 12.

What I have tried:

var d = new Date($("#<%=txtActDate.ClientID%>").val());
            var curr_date = d.getDate();                            
            if (packPeriod == 'Monthly') {

                 var curr_month = d.getMonth() + 1;
            var curr_year = d.getFullYear();
            var dates = curr_date + "/" + curr_month + "/" + curr_year;
              alert(dates );
            }
            else if (packPeriod == 'Quarterly') {
              var curr_month = d.getMonth() + 3;
            var curr_year = d.getFullYear();
            var dates = curr_date + "/" + curr_month + "/" + curr_year;
                alert(dates );
            }
            else if (packPeriod == 'HalfYearly') {
               var curr_month = d.getMonth() + 6;
            var curr_year = d.getFullYear();
            var dates = curr_date + "/" + curr_month + "/" + curr_year;
                alert(dates );
            }
            else if (packPeriod == 'Yearly') {
              var curr_month = d.getMonth() + 12;
            var curr_year = d.getFullYear();
            var dates = curr_date + "/" + curr_month + "/" + curr_year;
                alert(dates );
            }