One of my clients is using your plugin in their site, and it looks like you're overwriting any other cron schedules that other plugins try to implement. The issue is with the add_crons function. You should be accepting a $schedules array and then you would add your schedules to that array and then return it. Here's a revised add_crons function that fixes this issue:
function add_crons($schedules) {
$schedules['fifteenmins'] = array('display' => __( 'Every 15 minutes' ), 'interval' => 900);
$schedules['halfhour'] = array('display' => __( 'Every half hour' ), 'interval' => 1800);
$schedules['fourdaily'] = array('display' => __( 'Four times a day' ), 'interval' => 21600);
return $schedules;
}http://wordpress.org/extend/plugins/electric-studio-auto-expire-post/
