validar formato fecha

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

Dim fecha As String = tbfecha.Text.Trim

Dim formats() As String = {"d-MM-yyyy", "dd-MM-yyyy",
      "dd-M-yyyy", "d-M-yyyy"} ' más de un formato

Dim format As String = "dd-MM-yyyy"
        Dim thisDT As DateTime = Nothing

        If DateTime.TryParseExact(fecha, format, Globalization.CultureInfo.InvariantCulture, Globalization.DateTimeStyles.None, thisDT) Then

            MessageBox.Show("ok!")

        Else

            MessageBox.Show("no!")
        End If
End Sub

fuente: https://stackoverflow.com/questions/24841806/validate-the-date-format-in-vb-net