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add_filter( 'manage_edit-post_columns', 'aco_last_modified_admin_column' );

// Create the last modified column
function aco_last_modified_admin_column( $columns ) {
  $columns['modified-last'] =__( 'Last Modified', 'aco' );
  return $columns;
}

add_filter( 'manage_edit-post_sortable_columns', 'aco_sortable_last_modified_column' );

// Allow that content to be sortable by modified time information
function aco_sortable_last_modified_column( $columns ) {
  $columns['modified-last'] = 'modified';
  return $columns;
}

add_action( 'manage_posts_custom_column', 'aco_last_modified_admin_column_content', 10, 2 );

// Format the output
function aco_last_modified_admin_column_content( $column_name, $post_id ) {

  // Do not continue if this is not the modified column
  if ( 'modified-last' != $column_name )
    return;

  $modified_date   = the_modified_date( 'Y/m/d - g:i A' );
  $modified_author = get_the_modified_author();

  echo $modified_date;
  echo '<br>';
  echo '<strong>' . $modified_author . '</strong>';

}