Add the date of the invoice on credit memo

Hello, how to write the date of invoice instead of "completed" ?  thank you

    $order->content['order_number'] = array(
      '#markup' => t('Facture N° : @id du @date', array('@id' => $order->order_number, '@date' => date('d/m/Y', $order->completed))),
    );
    $order->content['order_id'] = array(
      '#markup' => t('Commande N° : @id du @date', array('@id' => $order->order_id, '@date' => date('d/m/Y', $order->created))),
    );
    $order->content['invoice_logo'] = array(
      '#value' => variable_get('commerce_billy_pdf_logo', 'misc/druplicon.png'),
    );