<div class="post-content">
<?php
$content = apply_filters('the_content', $post->post_content);
$save = explode("</p>", $content);
$count = 1;
foreach ($save as $a){
if ($count == 12){
   if (strcmp( date("Y-m-d", strtotime("-10 day")), get_the_time("Y-m-d") ) > 0) {
   ?>
AD CODE
   <?php
   }

}
echo $a."</p>";
$count = $count + 1;
}
?>
                <?php wp_link_pages(); ?>
            </div><!-- post-content -->