crontab was good enough for your gramma but not for Apple, circa 21st century.
Paul’s Time Sink | cron jobs with launchd:
In this post, I described ClamXav and how it will install a cron job to scan files as well as updating the virus definitions. In Mac OS X 10.4, Apple has created launchd to replace crond, init, and xinitd. So I wanted to replace with the cron jobs with Launch Actions.
The Launchd Editor he links to is worth a look, as well, and likely worth the $5 shareware fee.