Possibility to use order "changed date" in invoice receipt

First of all: thanks for this module. I've been trying a lot of approaches to generate invoices with DC and this module in combination with commerce_order_counter seems the most elegant to me.

We use this module for invoices and according to the law here (in Belgium) we have to put the date a product is delivered or when it got paid on an invoice. This could be a problem if you use the $order->created value on the invoice, since in some cases the customer payment happens days after the order got created (eg in case of a bank transfer) and the goods are also not delivered before payment. In our case I would prefer to use the date when an admin sets the order to completed on invoices, so I made a small patch to use $order->changed in the invoice. It's only one extra line of code, so it might be useful to include this in case other people would like to use that date?