ถ้าเราเลือกวันที่ใน datetimepicker มันจะ error ค่ะ แต่ถ้าเราไม่เลือกวันที่ มันก็ให้บันทึกข้อมูลได้ ทำไงดีค่ะ

Private Sub btnSave_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSave.Click
        Dim conn As New SqlClient.SqlConnection()
        Dim cmd As New SqlClient.SqlCommand()
        Dim a As String
        Dim cmdtxt As String
        a = ("รหัสสินค้า : " & txtserialno.Text & "") & vbNewLine
        a &= ("ชื่อสินค้า : " & txtname.Text & "") & vbNewLine
        a &= ("หน่วยสินค้า : " & cmbunit.Text & "") & vbNewLine
        a &= ("ราคาขาย/หน่วย : " & txtsaleprice.Text & "") & vbNewLine
        a &= ("ราคาทุน/หน่วย : " & txtbaseprice.Text & "") & vbNewLine
        a &= ("จำนวนสินค้าถึงจุดสั่งซื้อ : " & txtbaypoint.Text & "") & vbNewLine
        a &= ("คำแนะนำการใช้งาน : " & txtintroduction.Text & "") & vbNewLine
        a &= ("สรรพคุณ : " & txtindication.Text & "") & vbNewLine
        a &= ("คำเตือน : " & txtwarning.Text & "") & vbNewLine
        a &= ("ปริมาณสินค้าคงคลัง : " & txttotalstock.Text & "") & vbNewLine
        a &= ("ประเภทสินค้า : " & cmbtype.Text & "") & vbNewLine
        a &= ("วันที่ผลิต : " & dtpmfd.Text & "") & vbNewLine
        a &= ("วันหมดอายุ : " & dtpexp.Text & "")
        MessageBox.Show(a, "แสดงผล", MessageBoxButtons.OK, MessageBoxIcon.Information)
        conn.ConnectionString = "Data Source=.\SQLEXPRESS;AttachDbFilename='C:\Documents and Settings\User\My Documents\PROJECT MORYA.mdf';Integrated Security=True;Connect Timeout=30;User Instance=True"
        conn.Open()
        If Adddata = 1 Then
            cmdtxt = "insert into product (serialno,name,unit,saleprice,baseprice,baypoint,introduction,indication,warning,totalstock,type,mfd,exp) Values ('" & txtserialno.Text & "','" & txtname.Text & "','" & cmbunit.Text & "','" & txtsaleprice.Text & "','" & txtbaseprice.Text & "','" & txtbaypoint.Text & "','" & txtintroduction.Text & "','" & txtindication.Text & "','" & txtwarning.Text & "','" & txttotalstock.Text & "','" & cmbtype.SelectedIndex & "','" & dtpmfd.Value & "','" & dtpexp.Value & "')"
        Else
            cmdtxt = "update product set serialno = ('" & txtserialno.Text & "'),name = ('" & txtname.Text & "'),unit = ('" & cmbunit.Text & "'),saleprice = ('" & txtsaleprice.Text & "'),baseprice = ('" & txtbaseprice.Text & "'),baypoint = ('" & txtbaypoint.Text & "'),introduction = ('" & txtintroduction.Text & "'),indication = ('" & txtindication.Text & "'),warning = ('" & txtwarning.Text & "'),totalstock = ('" & txttotalstock.Text & "'),type = ('" & cmbtype.SelectedIndex & "'),mfd = ('" & dtpmfd.Value & "'),exp = ('" & dtpexp.Value & "') where serialno = '" & txtserialno.Text & "'"
        End If
        'MessageBox.Show("'" & txtid.Text & "','" & txtname.Text & "','" & txtaddress.Text & "', '" & txttel.Text & "','" & txtfax.Text & "','" & txtcomment.Text & "'", "ตรวจสอบ", MessageBoxButtons.OK, MessageBoxIcon.Question)
        cmd.Connection = conn
        cmd.CommandText = cmdTXT
        cmd.ExecuteNonQuery()
        conn.Close()
        MessageBox.Show("บันทึกข้อมูลเรียบร้อยแล้ว ", "บันทึกข้อมูล", MessageBoxButtons.OK, MessageBoxIcon.Information)
        btnAdd.Enabled = True
        btnSave.Enabled = False
        btnDelete.Enabled = True
        btnExit.Enabled = True
        btnEdit.Enabled = True

        txtserialno.Text = ""
        txtname.Text = ""
        cmbunit.Text = ""
        txtsaleprice.Text = ""
        txtbaseprice.Text = ""
        txtbaypoint.Text = ""
        txtintroduction.Text = ""
        txtindication.Text = ""
        txtwarning.Text = ""
        txttotalstock.Text = ""
        cmbtype.Text = ""
        dtpmfd.Text = ""
        dtpexp.Text = ""

        txtserialno.Text = ""
        txtname.Text = ""
        cmbunit.Text = ""
        txtsaleprice.Text = ""
        txtbaseprice.Text = ""
        txtbaypoint.Text = ""
        txtintroduction.Text = ""
        txtindication.Text = ""
        txtwarning.Text = ""
        txttotalstock.Text = ""
        cmbtype.Text = ""
        dtpmfd.Text = ""
        dtpexp.Text = ""

        txtserialno.Enabled = True
        txtname.Enabled = True
        cmbunit.Enabled = True
        txtsaleprice.Enabled = True
        txtbaseprice.Enabled = True
        txtbaypoint.Enabled = True
        txtintroduction.Enabled = True
        txtindication.Enabled = True
        txtwarning.Enabled = True
        txttotalstock.Enabled = True
        cmbtype.Enabled = True
        dtpmfd.Enabled = True
        dtpexp.Enabled = True

        txtserialno.Focus()
    End Sub