Add the custom email frequency for WPJM Job Alerts

wpjm-job-alerts-frequency.php

function htdat_job_manager_alerts_alert_schedules ( $schedules ) {
    $schedules['monthly'] = array(
        'interval' => 86400 * 30,
        'display'  => __( 'Monthly', 'wp-job-manager-alerts' )
    );

    return $schedules;
}

add_filter('job_manager_alerts_alert_schedules', 'htdat_job_manager_alerts_alert_schedules');