how to concatinate my html dropdownlistfor id values of day month time into single variable and should insert into database

@Html.DropDownListFor(model => model.Day, listItems, "Day",new {@Id="ddlDay", style = "width: 70px;" })
              @Html.DropDownListFor(model=>model.Month, listItemsMonth,"Month",new{@Id="ddlMonth",style = "width: 90px;"})
              @Html.DropDownList("Years", new SelectList(ViewBag.Years as System.Collections.IEnumerable, Model.Year), new {@Id="ddlYear",style = "width: 70px;" })

public DateTime Day { get; set; }
       public DateTime Month { get; set; }
       public DateTime Year { get; set; }
       public DateTime DateofBirth { get; set; }

public ActionResult AddUsers()
        {
            }

My db i have column as Datetime