public function getIssueTime()
    {
        $date = null;
        if(isset($this->_claims['iat']))
        {
            $value = $this->_claims['iat'];
            $date  = new DateTime('@'.$value);

            if ($date === false) {
                throw new RuntimeException(sprintf('The token issue time is not parseable (%s).', $value));
            }
        };

        return $date;
    }