รบกวนถามเรื่องการคำนวณวันเดือนปีใน vb หน่อยครับ เป็นโปรแกรมซื้อขายและเคลมสินค้า

Sub CH_Serial_No()
        Dim xx As Boolean = False
        Try


            Dim c_row As Integer = 0
            Dim i As Integer
            Dim add As Double = 0

            Dim sql As String = ""
            Dim conn2 As New SqlConnection _
                  (M_ConnStr.strConn)
            Dim Command_Adp2 As SqlCommand
            Dim Sql_Reader As SqlDataReader
            sql = "SELECT *  FROM tv_Bill where   Serial_No = '" & TextBox13.Text & "'    "
            conn2.Open()
            Command_Adp2 = New SqlCommand(sql, conn2)
            Sql_Reader = Command_Adp2.ExecuteReader()
            If Sql_Reader.Read() Then
                If Sql_Reader.Item("Pro_name").ToString() <> "" Then
                    Sql_Reader.Item("Pro_name").ToString()
                    TextBox12.Text = Sql_Reader.Item("Brand_name").ToString()
                    txt_Name_pro.Text = Sql_Reader.Item("Pro_name").ToString()
                    TextBox2.Text = Sql_Reader.Item("Date_Month").ToString()

                    DateTimePicker1.Visible = True
                    DateTimePicker2.Visible = True

                   
                    DateTimePicker1.Value = CDate(Sql_Reader.Item("OderCus_ProDate").ToString())
                    DateTimePicker2.Value = (บรรทัดนี้คือค่าของ TextBox1.Text + TextBox2.Text เขียนไม่ถูก)

                    TextBox1.Text = DateTimePicker1.Text
                    TextBox3.Text = DateTimePicker2.Text

                Else

                    MessageBox.Show("ไม่พบ Serial_No ")
                    TextBox12.Text = ""
                    txt_Name_pro.Text = ""

                    TextBox2.Text = ""
                    TextBox7.Text = ""
                    TextBox1.Text = ""
                    TextBox3.Text = ""
                    TextBox5.Text = ""
                    TextBox13.Focus()

                End If