public function add($string)
    {
        $this->timestamp = $this->getTimestamp() + $this->evaluateDateString($string);

        return $this;
    }
public function add($string)
    {
        $this->timestamp = $this->getTimestamp() + $this->evaluateDateString($string);

        return $this;
    }
public function add ( $ string ) { $ this -> timestamp = $ this -> getTimestamp ( ) + $ this -> evaluateDateString ( $ string ) ; return $ this ; }
Add a specific period

@param string $string The period to add

@return DateTime
Add a specific period