Custom Cron

I’ve seen your documentation on custom cron and I already know how to do this but I’m not sure how to get the plugin to pick up the new cron?

I’m trying to get a specific feed source to work on a custom processing interval of every 3 days

add_filter( 'cron_schedules', 'cron_add_weekly' );

function cron_add_weekly( $schedules ) {
$schedules['weekly'] = array(
'interval' => 259200,
'display' => __( 'Every 3 Days' )
);
return $schedules;
}

I’m not seeing the new cron option in the list of intervals on the individual feeds nor in the settings, code is being added via functions.php