Google API Client refresh OAuth2 token - php

Question

I use Google API to retrieve analytics data and display them on my website. it worked fine for a month, but the last four days I get this error :
Error refreshing the OAuth2 token, message: '{ "error" : "invalid_grant" }'
Here is the code :
<?php
require_once ('src/Google/autoload.php');
$client = new Google_Client();
$client->setApplicationName( 'GAFetchData' );
$client->setAssertionCredentials(
new Google_Auth_AssertionCredentials(
// Google client ID email address
'xxxxx-xxxxxxxxxx#developer.gserviceaccount.com',
array('https://www.googleapis.com/auth/analytics.readonly'),
file_get_contents( $Path.'gapii/'.'GAFetchData-xxxxxxxx.p12' )
)
);
$client->setClientId( 'xxxxx-xxxxxxxxxx.apps.googleusercontent.com' );
$client->setAccessType( 'offline_access' );
$analytics = new Google_Service_Analytics( $client );
$analytics_id = 'ga:xxxxxx';
try {
$optParams = array();
$metrics = 'ga:pageviews';
$start_date = '7daysAgo';
$end_date = 'today';
$optParams['dimensions'] = 'ga:pagePath';
$optParams['filters'] = 'ga:pagePath!#=rech;';
$optParams['max-results'] = '10000';
$result = $analytics->data_ga->get( $analytics_id, $start_date, $end_date, $metrics, $optParams);
if( $result->getRows() ) { $items = $result->getRows(); }
}
catch(Exception $e) { echo $e->getMessage();}
?>

Answer 1

Issue solved !
It was caused by the server's clock, I just synchronise it with a NTP server using the following command (Ubuntu in my case) :
sudo ntpdate 3.pool.ntp.org
You will find a list of time servers from the
public NTP time server list
hope that helps !