Adds hidden hatom tags to WordPress posts after the content block. They're not visible to humans but parsers will still see and parse them

add_hatom_tags.php

<?php 
/**
 * Adds hidden hatom tags to WordPress posts after the content block. 
 * They're not visible to humans but parsers will still see and parse them.
 *
 * @link https://imelgrat.me/wordpress/hentry-fix-errors-wordpress/
 *
 * @param string The post content
 */
function add_hatom_tags($content) 
{
if (is_home() || is_singular() || is_archive() ) {
        $content .= '<div class="hatom-extra" style="display:none;visibility:hidden;"><span class="entry-title">'.get_the_title().'</span> was last modified: <span class="updated"> '.get_the_modified_time('F jS, Y').'</span> by <span class="author vcard"><span class="fn">'.get_the_author().'</span></span></div>';
    }
    return $content;
}
add_filter('the_content', 'add_hatom_tags');

// This filter adds the entry-content hAtom tag, showing where the full content block is.
function add_hatom_post_content ($content) {
  if ( in_the_loop() && !is_page() ) {
    $content = '<span class="entry-content">'.$content.'</span>';
  }
  return $content;
}

add_filter( 'the_content', 'add_hatom_post_content');