I have already looked the document you suggested and unfortunately it doesn't really help. The documentation simply describes how to run the cronjobs and studying the cronjobs/notification.php and runcronjobs.php also just executes the events and event handlers.
After studying the above scripts (the best I could), I managed to add the event in the database with a specific send_date. But my handler does not seem to want to process these events.
There is lack of information on how to create custom handlers to process events in various ways (specific send_date). The handler I am using is a copy of the publishNotificationEventHandler. Which processes events of newly published objects. This may be the reason why it is not processing events with a send_date.
If only there was some documentation/examples of how to process events with a specific send_date. I think that would help.
I have run out of ideas now and really tempted to just create a php script outside of ezpublish and process my events in the database. But I would much rather use ezpublish to do this.