try this code
Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged
If (TextBox1.Text.Length = 2 Or TextBox1.Text.Length = 5) Then
TextBox1.Text = TextBox1.Text + "/"
TextBox1.SelectionStart = TextBox1.Text.Length
End If
If (TextBox1.Text.Length = 8) Then
Dim date_str As String = TextBox1.Text
Dim split_date As String() = date_str.Split("/")
Dim thisDate As DateTime = New DateTime(Convert.ToInt32("20" + split_date(2)), _
Convert.ToInt32(split_date(1)), _
Convert.ToInt32(split_date(0)))
TextBox1.Text = thisDate.ToString("d/MM/yyyy")
End If
End Sub