add custom weekly, monthly and 30 minutes cron time schedules via cron_schedules filter hook

custom_cron_scedule_intervals.php

<?php
function wptc_add_intervals( $schedules ) {
	// add a 'weekly' interval
	if( ! isset( $schedules['weekly'] ) ) {
		$schedules['weekly'] = array(
			'interval' => 604800, // seconds in a week
			'display' => __('Once Weekly')
		);
	}
	// add a 'monthly' interval
	if( ! isset( $schedules['monthly'] ) ) {
		$schedules['monthly'] = array(
			'interval' => 2635200, // seconds in a month
			'display' => __('Once a month')
		);
	}
	// add a '30min' interval
	if( ! isset( $schedules['30min'] ) ) {
		$schedules['30min'] = array(
		    'interval' => 1800, // seconds in 30 minutes or use 30*60
		    'display' => __('Once Every 30 minutes')
		);
	}
		
	return $schedules;
}
add_filter( 'cron_schedules', 'wptc_add_intervals' );