Thanks to the post "How to add a CRON job" by Sabrosa-2015, my problem is solved.
The final solution was for me, under root :
1 - modifying the /etc/init.d/jexec file by inserting the line "#Required-Stop : $local_fs" after the line beginning by "#Required_Start".
2 - adding the link S12bootclock --> /etc/init.d/boot.clock.
3 - activating the cron service by typing "chkconfig cron on".
4 - desactivating the ntp service by typing "chkconfig ntp off".
5 - creating a script called sntp_synchro in the directory /etc/cron_daily, containing the following line :
/usr/bin/sntp -V -P no -r server_name >> /var/log/sntp 2>&1
6 - rebooting the Terminal.
Please note that you will possibly need to modify manually the /etc/sysconfig.clock to set your TIMEZONE.