protected function getModifiedSince() : \DateTime
    {
        if (is_null($this->since)) {
            $this->since = new \DateTime('@0');
        }

        return $this->since;
    }
protected function getModifiedSince() : \DateTime
    {
        if (is_null($this->since)) {
            $this->since = new \DateTime('@0');
        }

        return $this->since;
    }
protected function getModifiedSince ( ) : \ DateTime { if ( is_null ( $ this -> since ) ) { $ this -> since = new \ DateTime ( '@0' ) ; } return $ this -> since ; }
Extract the 'If-Modified-Since' value from the headers.

@return \DateTime
Extract the If - Modified - Since value from the headers .