Private Sub Workbook_Open()
Dim lngCounter As Long
Sheets(1).Select
For lngCounter = 2 To 20 Step 1 'Schleife Anfang
    DoEvents
    If IsDate(Cells(lngCounter, 7)) Then
        If DateDiff("d", Cells(lngCounter, 7).Value, Date) >= -14 And _
          DateDiff("d", Cells(lngCounter, 7).Value, Date) < 0 Then
            MsgBox "Ihr TÜV für das ist fällig", vbInformation, "Achtung" & _
            "Autofahrer"
        End If
    End If
Next lngCounter
End Sub