timestamp is "-1"

[icepigss]

$config = \Kafka\ProducerConfig::getInstance();
$config->setMetadataRefreshIntervalMs(10000);
$config->setMetadataBrokerList($hostsList);
$config->setBrokerVersion('1.0.0');
$config->setRequiredAck(1);
$config->setClientId($clientId ?: 'kafka-php');
$config->setProduceInterval(500);
$this->_producer = new \Kafka\Producer();

$ret = $this->_producer->send([ [ 'topic' => 'hello, 'value' => $data, 'key' => ''] ]);

I don't know where something went wrong.
I got my message but it's field "timestamp" is "-1".

[pravinkucha]

explicitly pass message timestamp