cron_schedules WordPress Filter Hook

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The filter accepts an array of non-default cron schedules in arrays (an array of arrays). The outer array has a key that is the name of the schedule (for example, ‘weekly’). The value is an array with two keys, one is ‘interval’ and the other is ‘display’.

The ‘interval’ is a number in seconds of when the cron job shall run. So, for a hourly schedule, the ‘interval’ value would be 3600 or 60*60. For for a weekly schedule, the ‘interval’ value would be 60*60*24*7 or 604800.

The ‘display’ is the description of the non-default cron schedules. For the ‘weekly’ key, the ‘display’ may be __(‘Once Weekly’).

Why is this important?

When scheduling your own actions to run using the WordPress Cron service, you have to specify which interval WordPress should use. WordPress has its own, limited, default set of intervals, or “schedules”, including ‘hourly’, ‘twicedaily’, and ‘daily’. This filter allows you to add your own intervals to the default set.

For your plugin, you will be passed an array, you can easily add a weekly schedule by doing something like:

function my_add_weekly( $schedules ) {
	// add a 'weekly' schedule to the existing set
	$schedules['weekly'] = array(
		'interval' => 604800,
		'display' => __('Once Weekly')
	);
	return $schedules;
}
add_filter( 'cron_schedules', 'my_add_weekly' ); 

Adding multiple intervals works similarly:

function my_add_intervals($schedules) {
	// add a 'weekly' interval
	$schedules['weekly'] = array(
		'interval' => 604800,
		'display' => __('Once Weekly')
	);
	$schedules['monthly'] = array(
		'interval' => 2635200,
		'display' => __('Once a month')
	);
	return $schedules;
}
add_filter( 'cron_schedules', 'my_add_intervals'); 

Be sure to add your schedule to the passed array, as shown in the example. If you simply return only your own schedule array then you will potentially delete schedules created by other plugins.