Not a crontab solution but have a look at iCal alarms. I know they can be set to run when iCal isn't itself open but I don't know if they run when that user is not logged in.
XDA Developers on MSN
I automated my entire backup strategy with Zerobyte and a cron job
Zerobyte is one of my favorite backup tools out there. It uses Restic for the actual backup engine and adds an intuitive web ...
How-To Geek on MSN
I automated Linux backups with a simple bash script and cron (and it’s better than a GUI)
Skip one-click backup apps. This rsync script gives you full control over what gets saved and when, plus logs and a few hard-won lessons.
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