public function byYears(int $years): Date
    {
        return $this->yearTo($this->originalDate->year() + $years);
    }
public function byYears(int $years): Date
    {
        return $this->yearTo($this->originalDate->year() + $years);
    }
public function byYears ( int $ years ) : Date { return $ this -> yearTo ( $ this -> originalDate -> year ( ) + $ years ) ; }
changes date by given amount of years

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