add_filter('the_content', 'post_content');
function post_content($content) {
  $out = '<p>Yazı sonunda gözükecek paragraf buraya.</p>';
  return $content.$out;
}

add_filter('the_content', 'post_content');
function post_content($content) {
  global $post;
  $post_timestamp = get_post_timestamp($post->id);
  $out = '';
  
  if($post_timestamp > 1607555170)
  $out = '<p>Sonuna eklenecek yazı buraya.</p>';

  return $content.$out;
}