Authentication on google: OAuth2 keeps returning 'invalid_grant'

I started to configure google calendar on my new application. I almost made an exact copy of the authentication code displayed at google developers ( https://developers.google.com/google-apps/calendar/instantiate ), but i keep getting the following error:

Error fetching OAuth2 access token, message: 'invalid_grant'

I'm currently using Fork-CMS ( http://www.fork-cms.com ), a young lightweigth CMS. I correctly configured the config.php file of the google-api-php-client. (client-id, client-secret, redirect-uri, development key,...) and the redirect uri is correctly set on the google api's console. My code looks as follows:

<?php

/**
* This is a widget with a calendar implementation.
*
* @package       frontend
* @subpackage    events
*
* @author        Michiel Vlaminck <[email protected]>
*/
class FrontendEventsWidgetCalendar extends FrontendBaseWidget
{

    private $events = array();
    private $authUrl = array();

    /**
    * Execute the extra
    *
    * @return    void
    */
    public function execute()
    {      
        // call parent
        parent::execute();

        // load template
        $this->loadTemplate();

        // get data
        $this->getData();

        // parse
        $this->parse();
    }


    /**
    * Get the data from Google Calendar
    * This method is only executed if the template isn't cached
    *
    * @return    void
    */
    private function getData()
    {
        require_once PATH_LIBRARY . '/external/google-api-php-client/src/apiClient.php';
        require_once PATH_LIBRARY . '/external/google-api-php-client/src/contrib/apiCalendarService.php';

        $client = new apiClient();

        $service = new apiCalendarService($client);

        if (isset($_SESSION['oauth_access_token'])) {
            $client->setAccessToken($_SESSION['oauth_access_token']);
        } else {
            $token = $client->authenticate();
            $_SESSION['oauth_access_token'] = $token;
        }

        if ($client->getAccessToken()) {

            $calId = FrontendEventsModel::getCalendarId((int) $this->data['id']);
            $calId = $calId[0]['calendar_id'];

            $events = $service->events->listEvents($calId);
            $this->events = $events['items'];

            $_SESSION['oauth_access_token'] = $client->getAccessToken();

        } else {
            $this->authUrl = $client->createAuthUrl();
        }
    }


    /**
    * Parse
    *
    * @return    void
    */
    private function parse()
    {
        $this->tpl->assign('events', $this->events);
        $this->tpl->assign('authUrl', $this->authUrl);
    }
}

?>

When I open this widget-page for the first time, I get directed to google to authenticate the application. When I agree, I get redirected to my application and that's the point where I'm getting:

apiAuthException » Main

Message Error fetching OAuth2 access token, message: 'invalid_grant'
File    C:\wamp\www\Officevibes\library/external\google-api-php-client\src\auth\apiOAuth2.php
Line    105
Date    Thu, 05 Apr 2012 08:34:47 +0000
URL http://localhost/calendar?code=4/YPUpFklKvhEeTcMm4moRth3x49oe
Referring URL   (Unknown)
Request Method  GET
User-agent  Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.19 (KHTML, like Gecko) Chrome/18.0.1025.142 Safari/535.19

You should reuse the access token you get after the first successful authentication. You will get an invalid_grant error if your previous token has not expired yet. Cache it somewhere so you can reuse it.

I was having a similar problem caused by the time on my server being incorrect. Make sure your system clock is synchronised.

Go to your Google API Console ( https://code.google.com/apis/console/ ) and revoke your Client Secret under Client ID for installed applications.

Be sure to also update your code with the new Client Secret

1. Go to security.google.com
2. Revoke access
3. visit the authentication url again
4. you will now get a new refreshtoken

You'll also receive this error if you mistakenly try to authenticate your ID Token, rather than your Access Token.

So don't be like me - Make sure you pass the correct token into your code!