A cron job allows you to automatically set a function to run at a set time or interval. WordPress comes with an in-built function wp_schedule_event().

WordPress Schedule Event

The function wp_schedule_event() allows you schedule a hook to run an action to schedule a function to run at a set interval.

<?php wp_schedule_event($timestamp, $recurrence, $hook, $args); ?>

Schedule An Hourly Event In Plugin

To schedule an hourly event within a plugin you will schedule a function on activation of the plugin.

register_activation_hook(__FILE__, 'my_activation');
add_action('my_hourly_event', 'do_this_hourly');
function my_activation() {
	wp_schedule_event( current_time( 'timestamp' ), 'hourly', 'my_hourly_event');
}
function do_this_hourly() {
	// do something every hour
}

If you want to deactivate the schedule event then use this.

register_deactivation_hook(__FILE__, 'my_deactivation');
function my_deactivation() {
	wp_clear_scheduled_hook('my_hourly_event');
}

Schedule hourly Event In Functions.php

This doesn't rely on activation of the plugin as it will run inside the functions.php file.

add_action('my_hourly_event', 'do_this_hourly');
function my_activation() {
	if ( !wp_next_scheduled( 'my_hourly_event' ) ) {
		wp_schedule_event( current_time( 'timestamp' ), 'hourly', 'my_hourly_event');
	}
}
add_action('wp', 'my_activation');
function do_this_hourly() {
	// do something every hour
}