problem in saving the date

[mshravs]

here below is the code for saving the form

Protected Sub save_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles save.Click
        Dim ConnStr As String
        Dim visitors_name, officer_to_visit, purpose, date_of_visit, time_in, time_out As String
        Dim serial_no, year, floor As String
        Dim photo As Byte
        ConnStr = ConfigurationManager.ConnectionStrings("gate").ConnectionString()
        Dim conn As New SqlConnection(ConnStr)
        conn.Open()
        serial_no = serialno.Text
        year = years.Text
        visitors_name = visitorsname.Text
        officer_to_visit = officertovisit.Text
        purpose = pur.Text
        date_of_visit = CDate(dateofvisit.Text)
        floor = wing.Text
        time_in = inout.Text
        time_out = timeout.Text
        Dim insert As New SqlClient.SqlCommand("insert into visitors_pass (serial_no,year ,visitors_name ,officer_to_visit,purpose,date_of_visit,floor,time_in,time_out,photo) Values('" & serial_no & "' ,'" & year & "', '" & visitors_name & "','" & officer_to_visit & "','" & purpose & "',CONVERT(datetime ,'" & date_of_visit & "', 103),'" & floor & "','" & time_in & "','" & time_out & "','" & photo & "')", conn)
        insert.ExecuteNonQuery()
        conn.Close()
        MsgBox("record saved")
    End Sub

the problem is with saving of the date. when i try to save the number above 20 it is giving an exception as Conversion from string "23/12/2009" to type 'Date' is not valid why is this like this
plz reply and give a solution needed urgently
thnks a lot