Amazon MWS Order API时间戳必须遵循ISO8601

我正在使用Amazon MWS订单API (ListOrders)，我可以成功地在Amazon Scratchpad上运行它，但我得到以下错误

发送者

MalformedInput
时间戳必须遵循ISO8601

下面是我从一些Stackoverflow帖子中得到的php脚本

$base_url = "https://mws.amazonservices.com/Orders/2013-09-01";
$method = "POST";
$host = "mws.amazonservices.com";
$uri = "/Orders/2013-09-01";
$params = array(
    'AWSAccessKeyId' => "AWSAccessKeyId",
    'Action' => "ListOrders",
    'SellerId' => "SellerId",
    'SignatureMethod' => "HmacSHA256",
    'SignatureVersion' => "2",
    //'Timestamp'=> gmdate("Y-m-d'TH:i:s.''0''0''0''Z", time()),
    'Timestamp'=> gmdate("Y-m-d'TH:i:s'Z", time()),
    'Version'=> "2013-09-01",
    'MarketplaceId' => "MarketplaceId",
    'CreatedAfter'=>'2014-07-06T19%3A00%3A00Z',
    'CreatedBefore'=>'2014-07-08T19%3A00%3A00Z'
    );
// Sort the URL parameters
$url_parts = array();
foreach(array_keys($params) as $key)
    $url_parts[] = $key . "=" . str_replace('%7E', '~', rawurlencode($params[$key]));
sort($url_parts);
// Construct the string to sign
$url_string = implode("&", $url_parts);
$string_to_sign = "GET'nmws.amazonservices.com'n/Orders/2013-09-01'n" . $url_string;
// Sign the request
$signature = hash_hmac("sha256", $string_to_sign, AWS_SECRET_ACCESS_KEY, TRUE);
// Base64 encode the signature and make it URL safe
$signature = urlencode(base64_encode($signature));
$url = "https://mws.amazonservices.com/Orders/2013-09-01" . '?' . $url_string . "&Signature=" . $signature;
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL,$url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_TIMEOUT, 15);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
$response = curl_exec($ch);
$parsed_xml = simplexml_load_string($response);
print '<pre>';
print_r($response);