code: public function guard($value, Field $field)
      {
          Assertion::integer($value, null, $field->getName());
          Assertion::true(
              DateUtils::isValidTimestamp($value),
              sprintf('Field [%s] value [%d] is not a valid unix timestamp.', $field->getName(), $value),
              $field->getName()
          );
      }
docs: {@inheritdoc}