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

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

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

@param string $string The period

@return DateTime
Subtract a specific period