Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Shared/Date.php
public static function setDefaultTimezone($timeZone)
    {
        if ($timeZone = self::validateTimeZone($timeZone)) {
            self::$defaultTimeZone = $timeZone;

            return true;
        }

        return false;
    }
public static function setDefaultTimezone($timeZone)
    {
        if ($timeZone = self::validateTimeZone($timeZone)) {
            self::$defaultTimeZone = $timeZone;

            return true;
        }

        return false;
    }
Set the Default timezone to use for dates.

@param DateTimeZone|string $timeZone The timezone to set for all Excel datetimestamp to PHP DateTime Object conversions

@throws \Exception

@return bool Success or failure
@return bool Success or failure