Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Shared/TimeZone.php
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) ? $transitions[0]['offset'] : 0;
    }
public static function getTimeZoneAdjustment($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;
    }
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