Please try below code
int month = DateTime.ParseExact(Convert.ToString(ddlMonth.SelectedValue), "MMMM", CultureInfo.CurrentCulture).Month;
int day = Convert.ToInt32(ddlDay.SelectedValue);
int year=DateTime.Now.Year;
DateTime date = new DateTime(year,month, day);
//Use AddDays for add and substract days
date.AddDays(-1);
string str=String.Format("{0:m}", date);