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.

Michel

The link S12bootclock needs to be created in /etc/init.d/rc5.d.