Try this:
SqlParameter WarrantyTillparam = new SqlParameter("@WarantyTill", SqlDbType.Date);
DateTime yourDateTime = DateTime.ParseExact(txtwarrantytill.Text, "yyyyMMdd", CultureInfo.InvariantCulture);
WarrantyTillparam.Value = yourDateTime;
cmd.Parameters.Add(WarrantyTillparam);