static function getDaysDiff($startDate, $endDate, Settings\DateSettings $settings = null)
    {
        $format = $settings ? $settings->dateFromat : Settings\DefaultDateSettings::$defaultDateFromat;
        $start  = self::createFromFormat($format, $startDate);
        $end    = self::createFromFormat($format, $endDate);
        return $start && $end ? $start->diff($end)->format('%r%d') : false;
    }