@baardbaard WordPress internal cron is unreliable as it requires site traffic to fire. A more reliable and timely way to manage scheduled tasks is to disable Internal WP Cron and have the server handle timing.
In short, cron jobs should be left to cron.
Some of the background in this article may be helpful:
http://ben.lobaugh.net/blog/20787/wordpress-how-to-use-wp-cron