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The filter accepts an array of arrays. The outer array has a key that is the name of the schedule or 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 should run. So for 'hourly', the time is 3600 or 60*60. For weekly, the value would be 60*60*24*7 or 604800. The value of 'interval' would then be 604800.
The 'display' is the description. For the 'weekly' key, the 'display' would be __('Once Weekly').
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;
}