Conditionally Check For Old WordPress Posts Only

Taken from WordPress hack : How to display ads on old posts only.

function is_old_post($post_id = null){
  $days = 14;
  global $wp_query;
  if ( is_single() || is_page() ) {
    if( !$post_id ) {
      $post_id = $wp_query->post->ID;
    }
    $current_date = time();
    $offset = $days * 60 * 60 * 24;
    $post_id = get_post($post_id);
    $post_date = mysql2date('U', $post_id->post_date);
    $cunning_math = $post_date + $offset;
    $test = $current_date - $cunning_math;
    if( $test > 0 ) {
      $return = true;
    } else {
      $return = false;
    }
  } else {
    $return = false;
  }
  return $return;
}

Further functions can now be added to so that certain content (e.g. adverts) can be added to posts once they are a set time past their published date.