As Francisco already mentioned: If you need more flexibility (and security - because inside the container neither you nor a future QNAP firmware update can break anything) I would install crontab in a mini Linux container.
On QNAP you can install docker - on QNAP the app or GUI is called "container station" and then search directly for a mini Linux or crontab image.
If you have Container Station installed you can also install containers via the CLI e.g. docker pull okteto/crontab
.
The simple approach on the console of the NAS would be:
1] Edit /etc/config/crontab file and add your custom job. for example: 12 0 * * /bin/syncmyqnap.sh
2] Test crontab /etc/config/crontab
3] Restart crondaemon – crontab /etc/config/crontab && /etc/init.d/crond.sh restart
Check QNAP's wiki for details: https://wiki.qnap.com/wiki/Add_items_to_crontab