在Google Datastore中存储日期值

我正在尝试使用Google API PHP客户端在Google数据存储中存储日期值，但服务总是回复错误:

Invalid value for: Invalid format: "2014-08-18 12:40:52" is malformed at " 12:40:52 .

function create_entity($name, $property, $data) {
    $entity = new Google_Service_Datastore_Entity();
    $entity->setKey(createKey($name));
    $string_prop = new Google_Service_Datastore_Property();
    $string_prop->setStringValue($data);
    $string_prop->setIndexed(false);
    $time = date("Y-m-d H:i:s");
    $string_date = new Google_Service_Datastore_Property();
    $string_date->setDateTimeValue($time);
    $string_date->setIndexed(false);
    $property_map = [];
    $property_map[$property] = $string_prop;
    $property_map['date'] = $string_date;
    $entity->setProperties($property_map);
    return $entity;
}
function create_commit($name, $property, $data) {
    $entity = create_entity($name, $property, $data);
    $mutation = new Google_Service_Datastore_Mutation();
    $mutation->setUpsert([$entity]);
    $req = new Google_Service_Datastore_CommitRequest();
    $req->setMode('NON_TRANSACTIONAL');
    $req->setMutation($mutation);
    return $req;
}