public function byDays(int $days): Date
    {
        return $this->dayTo($this->originalDate->day() + $days);
    }
public function byDays(int $days): Date
    {
        return $this->dayTo($this->originalDate->day() + $days);
    }
public function byDays ( int $ days ) : Date { return $ this -> dayTo ( $ this -> originalDate -> day ( ) + $ days ) ; }
changes date by given amount of days

@api
@param   int  $days
@return  \stubbles\date\Date
changes date by given amount of days