On a new installed system cron does shutdown as wanted. I eeed a "kiosk System" and modified as described for example : https://itrig.de/index.php?url=archives/2309-Raspberry-Pi-3-Kiosk-Chromium-Autostart-im-Vollbildmodus-einrichten.html&serendipity[cview]=linear#c2435 and now cron shut down is not ...