Crontab would not be installed by default on CentOS 5 custom installation, as happened with my new prgmr.com server. So to install cron you would do a yum search cron and you are not quiet sure what to install.
To install crontab on centos
yum install -y vixie-cron.x86_64
Vixie cron is the cron software on CentOS 5.
The Vixie cron daemon for executing specified programs at set times.
Now you should be able to do crontab -e to add your cron.
sudo /etc/init.d/crond start
Now the cron tasks should be executed at the specified times.