public function transform($dateTime)
    {
        if (null === $dateTime) {
            return '';
        }

        if (!$dateTime instanceof \DateTime) {
            throw new TransformationFailedException('Expected a \DateTime.');
        }

        // convert time to UTC before passing it to the formatter
        $dateTime = clone $dateTime;

        if ('UTC' !== $this->inputTimezone) {
            $dateTime->setTimezone(new \DateTimeZone('UTC'));
        }

        $value = $this->getIntlDateFormatter()->format((int) $dateTime->format('U'));

        if (intl_get_error_code() != 0) {
            throw new TransformationFailedException(intl_get_error_message());
        }

        return $value;
    }