code: public function normalize($object, $format = null, array $context = array())
      {
          $dt = clone $object;
          $dt->setTimezone(new \DateTimeZone('UTC'));
  
          return $object->format(self::XSD_DATETIME_FORMAT);
      }
docs: Normalizes an object into a set of arrays/scalars
  
  @param object $object object to normalize
  @param string $format format the normalization result will be encoded as
  @param array  $context Context options for the normalizer
  
  @return string