code: private function convertToUtc(\DateTimeInterface $date)
      {
          if ($date->getOffset() !== 0) {
              if (null === $this->utc) {
                  $this->utc = new \DateTimeZone('UTC');
              }
  
              if ($date instanceof \DateTimeImmutable) {
                  $date = \DateTime::createFromFormat(
                      DateUtils::ISO8601_ZULU,
                      $date->format(DateUtils::ISO8601_ZULU)
                  );
              }
  
              $date->setTimezone($this->utc);
              $date = \DateTimeImmutable::createFromMutable($date);
          }
  
          return $date;
      }
docs: @param \DateTimeInterface $date
  
  @return \DateTimeInterface