[CentOS] Inquiry:How to make crontab job permanent even after server reboot?

Dear All
Please be informed that I have an CentOS 5 server and I need it to be
automatically rebooted at pre-specified times . To this end , I tried to set
it as its crontab job as the followings :
#crontab -e
30 15 * * * reboot
It got through but after the server first reboot the crontab job disappeared
. So I tried to set it in another way , as the followings :
set the cron list in /tmp/temp
add the following lines to /etc/rc.local :
crontab /tmp/temp
But still the crontab job will disappear after the server first reboot . Can
you please do me favor and let me know how can I make the crontab job
permanent on my CentOS server even after server reboot ?
Let me thank you in advance
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