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Well the issue with cronjobs is that you cannot make them easily without going to cpanel and doing something newbie unfriendly. For advanced users its the way to go, but for a Mod/Plugin doing cronjobs like this will confuse many people. A rule of thumb when making a Mod/Plugin is not to make the installation/modification process over-complicated.

Alternatively, you can set up something called scheduled tasks, or pseudo-cronjobs. All you need is to insert a script that runs whenever someone visits your site, it will check for scheduled tasks and will execute any tasks that are ready to run(or overdue). In an active site it will emulate real cronjobs very well, on an inactive or dev/demo site it may not work as expected though. Mysidia Adoptables v1.4.0 will introduce official scheduled tasks system, I am sure it will help many users who want time-based actions/events.