if (order.containsBackorderItems())
{
    //Fetch all items that has to be backordered from supplier
    items = order.backorderItems();

    //Get the "slowest" item, based on delivery calculation of the (active) supplier
    slowestItem = items.sortBy(function(item) {
        return item.supplier.delivery_calculation_time;
    }).first();

    //Set the order delivery date to the longest of the suppliers
    order.deliveryDate = slowestItem.supplier.delivery_calculation_time;
} else {
    order.deliveryDate = "today+3D";
}