MailPoet custom shortcode

e.g. a shortcode to display previous month:

 

// Custom MailPoet shortcode to show last month
add_filter('mailpoet_newsletter_shortcode', 'sws_mailpoet_shortcode_for_last_month', 10, 6);
function sws_mailpoet_shortcode_for_last_month($shortcode, $newsletter, $subscriber, $queue, $newsletter_body, $arguments) {
  // always return the shortcode if it doesn't match your own!
  if ($shortcode !== '[custom:last_month]') return $shortcode; 
  
  return date('F',strtotime('first day of -1 month'));
}