The cart produces a PDF invoice with an order date (the date the order was placed , as stored in the database)
I'm thinking along the lines of a wholesaler here (i've experienced this issue), so it may not be an issue for everyone but here goes:
sometimes, a customer may order a product - for example on 1st Jan - but not want delivery until 1st May
The order is processed when it is received so the date is set to 1st Jan - BUT, the customer will not want an invoice dated back to when the order was placed, they will want an invoice dated around the expected delivery date.
(I know I suggested a while ago that a custom textbox in customer order specifically for addditional PDF information would be a good idea) what i am suggesting here is that another custom textbox be provided in customer order page so that specific invoice dates could be applied if necessary.
In the above example, administrator might open the order around the end of April, enter the date eg 23rd April in the new textbox, then print the PDF invoice - it would have the order date, but it would also have a relevant invoice date.