[C#] Format ของ DateTimePicker. คืออะไรคับ ผมจะแสดงข้อมูลจาก textbox ผ่าน DataGrid /SQL S 2008

private void Save_detail_Click(object sender, EventArgs e)
        {
            string sql = "INSERT INTO tb_CheckIn (CK_Id,CK_Fname,CK_Lname,CK_Phone,CK_Address)VALUES('" + Check_Id.Text.Trim() + "','" + Check_RoomId.Text.Trim() +"','" + Check_Fname.Text.Trim() + "','" + Check_Lname.Text.Trim() + "','" + Check_Phone.Text.Trim() + "','" + Check_Address.Text.Trim() + "','" + Check_In.Value.Date.ToString() + "','" + Check_Out.Value.Date.ToString() + "')";
            
            SqlCommand com = new SqlCommand(sql, conn);
            com.ExecuteNonQuery();

            MessageBox.Show("บันทึกข้อมูลเรียบร้อย");
            Showdata();
            clearData();
            
        }

 '" + Check_In.Value.Date.ToString() + "','" + Check_Out.Value.Date.ToString() + "'