Research for automated email delivery for role: trainer

Hi all,

hope someone can help me with the research on  implementing a new function on a Moodle 1.9 installation.

I like to implement automated email delivery to the trainer, with predefined text, if a course date is expired. I wonder if this can be done via cron job. If yes - does anyone have any idea where to start the research, since I didn't found much info on editing the cron.php. Any other ideas how to archieve it or where to  start the research? Or wich other resources do you recommend to start from.

Thx in advance for any ideas on this topic.