try as
string date= txtusersurname.Text +"/" +dldbirthdate.Text + "/" + dldbirthmonth.Text+ "/"+ dldbirthyear.Text;
string str="insert into tablename values(@txtusername,@txtusersurname,@date)";
SqlCommand cmd=new SqlCommand(str,connection);
cmd.Parameters.AddWithValue("txtusername",txtusername.Text);
cmd.Parameters.AddWithValue("txtuserurname",txtuserurname.Text);
cmd.Parameters.AddWithValue("date",date);