custom module date formatter for publication date
The publication date module provides a function to define which formatters can be used. I added a patch, which we maintain locally, to add the defined "timestamp_ap_style" format to the options. Optionally, I could have added it inside a custom module, but the patch seemed to make more sense.

 function publication_date_field_formatter_info_alter(array &amp;$info) {
   $info['timestamp']['field_types'][] = 'published_at';
   $info['timestamp_ago']['field_types'][] = 'published_at';
   $info['timestamp_ap_style']['field_types'][] = 'published_at';
 }