Hello,
For that use the below code...in that first you have to make a date from your string field:
//Make a Date
//if 5(May) is Month and 6 is Date
string Getdate = "5/6";
DateTime date = Convert.ToDateTime(Getdate + "/1999");
return date.AddDays(15);
//It will return Date {05/21/1999 12:00:00 AM}