Date inserted as STRING but should be DATE or TIMESTAMP

Working with Data
3

My Controller Code:

$mongoData = array(
          'type' => $post['type'],  *//STRING TYPE*
          'product_data' => $post['event_data'], *//Object*
          'date_added' => $post['event_time'] *//STRING TYPE, But should be date/timestamp*
 );

        $mongoCollection = 'myCollection';

        $this->mongodb->insert($mongoCollection, $mongoData);

My MongoDB Library Code is:

$insert = $collection->insertOne($data);