public function addTimestampValidation() {
        // Add timestamp-based spam counter-measure
        $this->addElement(
            'hidden', self::TIMESTAMP_FIELD_KEY, array(
            'validators' => array(new Garp_Validate_Duration)
            )
        );
        // If the form is submitted, do not set the value.
        if (!$this->getValue(self::TIMESTAMP_FIELD_KEY)) {
            $now = time();
            $this->getElement(self::TIMESTAMP_FIELD_KEY)->setValue($now);
        }
    }