public function refreshToken($forceRefresh = false)
    {
        if (property_exists($this, "sdl") && $this->sdl == 0) {
            parent::__construct($this->xmlLoc, array('trace'=>1, 'exceptions'=>0));
        }
        try {
            $currentTime = new DateTime();
            if (is_null($this->getAuthTokenExpiration($this->tenantKey))) {
                $timeDiff = 0;
            } else {
                $timeDiff = $currentTime->diff($this->getAuthTokenExpiration($this->tenantKey))->format('%i');
                $timeDiff = $timeDiff  + (60 * $currentTime->diff($this->getAuthTokenExpiration($this->tenantKey))->format('%H'));
            }
            if (is_null($this->getAuthToken($this->tenantKey)) || ($timeDiff < 5) || $forceRefresh) {
                $url = $this->tenantKey == null
                        ? $this->baseAuthUrl."/v1/requestToken?legacy=1"
                        : $this->baseUrl."/provisioning/v1/tenants/{$this->tenantKey}/requestToken?legacy=1";

                $jsonRequest = new stdClass();
                $jsonRequest->clientId = $this->clientId;
                $jsonRequest->clientSecret = $this->clientSecret;
                $jsonRequest->accessType = "offline";
                if (!is_null($this->getRefreshToken($this->tenantKey))) {
                    $jsonRequest->refreshToken = $this->getRefreshToken($this->tenantKey);
                }
                $authResponse = ET_Util::restPost($url, json_encode($jsonRequest), $this);
                $authObject = json_decode($authResponse->body);
                //echo "auth:  \n";
                //print_r($authResponse);

                if ($authResponse && property_exists($authObject, "accessToken")) {
                    $dv = new DateInterval('PT'.$authObject->expiresIn.'S');
                    $newexpTime = new DateTime();
                    $this->setAuthToken($this->tenantKey, $authObject->accessToken, $newexpTime->add($dv));
                    $this->setInternalAuthToken($this->tenantKey, $authObject->legacyToken);
                    if (property_exists($authObject, 'refreshToken')) {
                        $this->setRefreshToken($this->tenantKey, $authObject->refreshToken);
                    }
                } else {
                    throw new Exception('Unable to validate App Keys(ClientID/ClientSecret) provided, requestToken response:'.$authResponse->body);
                }
            }
        } catch (Exception $e) {
            throw new Exception('Unable to validate App Keys(ClientID/ClientSecret) provided.: '.$e->getMessage());
        }
    }