Have added the order delivery date module so that the customer chooses the date on the checkout shipping page which inserts a session parameter of the order delivery date. I want to require that so the original code is:

if (zen_not_null($_POST['order_delivery_date'])) {
$_SESSION['order_delivery_date'] = zen_db_prepare_input($_POST['order_delivery_date']);
}
$order_delivery_date = $_SESSION['order_delivery_date'];

I just added this below that:

if (!$order_delivery_date) {
$messageStack->add('checkout_shipping', ERROR_PLEASE_CHOOSE_DELIVERY_DATE, 'error');
}
I've tried !$_SESSION['order_delivery_date'] and !$_POST['order_delivery_date'] as well but since none are working, I assume I've done something else wrong.

Is there a specific place in the header.php.php that an error message should go? Or is the use of check_shipping incorrect in that line? Forgive me, this is the first time I've ever tried adding to the message stack in my 5 plus years experience in zen cart!