code: public static function getTimeZoneAdjustment($timezone, $timestamp)
      {
          if ($timezone !== null) {
              if (!self::validateTimezone($timezone)) {
                  throw new PhpSpreadsheetException('Invalid timezone ' . $timezone);
              }
          } else {
              $timezone = self::$timezone;
          }
  
          if ($timezone == 'UST') {
              return 0;
          }
  
          $objTimezone = new DateTimeZone($timezone);
          $transitions = $objTimezone->getTransitions($timestamp, $timestamp);
  
          return (count($transitions) > 0) ? $transitions[0]['offset'] : 0;
      }
docs: Return the Timezone offset used for date/time conversions to/from UST
  This requires both the timezone and the calculated date/time to allow for local DST.
  
  @param string $timezone The timezone for finding the adjustment to UST
  @param int $timestamp PHP date/time value
  
  @throws PhpSpreadsheetException
  
  @return int Number of seconds for timezone adjustment