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));
    }