Now that we have the document editor, we should have a string for Invoice date (the date the Invoice was generated) and Credit Note date (the date the Credit Note was generated). These are not the same as the order date and should be linked to the Invoice / Credit Memo number generation which is different to the order number.
I believe there is an addon for this. I can't remember it top of my head, please check market place. i haven't switched to 4.4.x so I don't have this function yet.