Piwik\Date::adjustForTimezone PHP Method

    public static function adjustForTimezone($timestamp, $timezone)
    {
        if (empty($timezone)) {
            return $timestamp;
        }
        // manually adjust for UTC timezones
        $utcOffset = self::extractUtcOffset($timezone);
        if ($utcOffset !== false) {
            return self::addHourTo($timestamp, $utcOffset);
        }
        date_default_timezone_set($timezone);
        $datetime = date(self::DATE_TIME_FORMAT, $timestamp);
        date_default_timezone_set('UTC');
        return strtotime($datetime);
    }

 /**
  * The Multisites reports displays the first calendar date as the earliest day available for all websites.
  * Also, today is the later "today" available across all timezones.
  * @param array $siteIds Array of IDs for each site being displayed.
  * @return Date[] of two Date instances. First is the min-date & the second
  *               is the max date.
  * @ignore
  */
 public static function getMinMaxDateAcrossWebsites($siteIds)
 {
     $siteIds = self::getIdSitesFromIdSitesString($siteIds);
     $now = Date::now();
     $minDate = null;
     $maxDate = $now->subDay(1)->getTimestamp();
     foreach ($siteIds as $idsite) {
         // look for 'now' in the website's timezone
         $timezone = Site::getTimezoneFor($idsite);
         $date = Date::adjustForTimezone($now->getTimestamp(), $timezone);
         if ($date > $maxDate) {
             $maxDate = $date;
         }
         // look for the absolute minimum date
         $creationDate = Site::getCreationDateFor($idsite);
         $date = Date::adjustForTimezone(strtotime($creationDate), $timezone);
         if (is_null($minDate) || $date < $minDate) {
             $minDate = $date;
         }
     }
     return array(Date::factory($minDate), Date::factory($maxDate));
 }