I would guess that if you look at the formula in the date box is says "now()" correct?
This is an auto update to the date the PC is set to.
Simply type the date when you start a new invoice istead of using the auto-date option.
Select the box, format cell to a date cell of your choice, and type the date in. Then when you open it up, it will keep the original date.
Why not just keep a copy though in paper format so you have the original dates anyway, that's what I do?