code: public function getExpirationDate($currentDate)
      {
          $numberOfDays = Mage::helper('sheep_debug')->getPersistLifetime();
  
          return date(self::DATE_FORMAT, strtotime("-{$numberOfDays} days {$currentDate}"));
      }
docs: Removes configured number of days from specified date
  
  @param string $currentDate
  @return bool|string