Dim DTP_Clicked as boolean
 
Private Sub DateTimePicker1_ValueChanged(sender As System.Object, _
    e As System.EventArgs) Handles DateTimePicker1.ValueChanged
    DTP_clicked = true 'Wertänderung notieren
End Sub
 
'Der Handler des Save-Button
Private Sub Button1_Click(sender As System.Object, 
    e As System.EventArgs) Handles Button1.Click
 
    if not DTP_Clicked then 
       msgbox "Betätigen Sie das DTP-Control"
    else
       'Speichern ausführen
    end if
End Sub