This creates a recurring cron job in Wordpress. From http://wp.smashingmagazine.com/2013/10/16/schedule-events-using-wordpress-cron/

recurring_cron.php

//On plugin activation schedule our daily database backup 
register_activation_hook( __FILE__, 'wi_create_daily_backup_schedule' );
function wi_create_daily_backup_schedule(){
  //Use wp_next_scheduled to check if the event is already scheduled
  $timestamp = wp_next_scheduled( 'wi_create_daily_backup' );

  //If $timestamp == false schedule daily backups since it hasn't been done previously
  if( $timestamp == false ){
    //Schedule the event for right now, then to repeat daily using the hook 'wi_create_daily_backup'
    wp_schedule_event( time(), 'daily', 'wi_create_daily_backup' );
  }
}

//Hook our function , wi_create_backup(), into the action wi_create_daily_backup
add_action( 'wi_create_daily_backup', 'wi_create_backup' );
function wi_create_backup(){
  //Run code to create backup.
}

register_deactivation_hook( __FILE__, 'wi_remove_daily_backup_schedule' );
function wi_remove_daily_backup_schedule(){
  wp_clear_scheduled_hook( 'wi_create_daily_backup' );
}