Variation of if_modified_since WP plugin to updated Last-Modified based on most recent comment

gistfile1.php

/* Do Not Change Anything here */
add_action('template_redirect', 'ism_add_last_modified_header');

function ism_add_last_modified_header($headers) {

    //Check if we are in a single post of any type (archive pages has not modified date)
    if( is_singular() ) {
        $post_id = get_queried_object_id();
        if( $post_id ) {
            $comments = get_comments(array(
              'number' => 1,
              'orderby' => 'comment_date',
              'order' => 'desc',
              'post_id' => $post_id
            ));

            $comment_last_mod = date_create_from_format('Y-m-d H:i:s', $comments[0]->comment_date);
            $c_date = $comments[0]->comment_date;
            $post_last_mod = date_create_from_format('U', get_the_modified_time("U", $post_id));
            $dates = array($comment_last_mod, $post_last_mod);
            $newer = max(array($comment_last_mod, $post_last_mod));
            $date = $newer->format("D, d M Y H:i:s");
            header("Last-Modified: $date");
        }
    }

}

?>