<?php
function wp_custom_cron_job_monthly( $schedules ) {
    if(!isset($schedules['monthly'])){
        $schedules['monthly'] = array(
            'display' => __( 'Once monthly', 'textdomain' ),
            'interval' => 2635200,
        );
    }

    return $schedules;
}
add_filter( 'cron_schedules', 'custom_cron_job_monthly' );