PHP indexing documents in elasticsearch won't accept timestamp

Question

I am inserting a simple document in elasticSearch using PHP. The document is inserted but unfortunately the timestamp is not recorded, and no time stamp is added automatically.

here is a sample code :

<?php
require __DIR__ . '/vendor/autoload.php';
use Elasticsearch\ClientBuilder;
use Carbon\Carbon;

 if($_SERVER["REQUEST_METHOD"] == "POST") {
      // username and password sent from form 

      $firstName = $_POST['FirstName'];
      $lastName = $_POST['LastName']; 
      $address = $_POST['Address']; 

      $hosts = [
          'http://localhost:9200'
      ];
      $client = ClientBuilder::create()           // Instantiate a new ClientBuilder
                          ->setHosts($hosts)      // Set the hosts
                          ->build();              // Build the client object



      $myDateTime=Carbon::now();
      $params = [
          'index' => 'auditsvilles',
          'type' => 'logs',
          'id' => 'id0001',
          'timestamp' => $myDateTime->toIso8601String(),
          'body' => ['FirstName' => $firstName, 'LastName' => $lastName]
      ];

      $response = $client->index($params);
      print_r($response);
  }
?>

I have searched quite extensively, and found someinputs about using Iso8601, but it didn't change.