I beg to differ on the word "actual", I would rather use "current" date (at the time you are typing, certainly.)  

I could be mistaken, though, but for the time being this is what I think.   "Actual" being a false cognate in this case.

Thus,  my version  goes like this:

The program doesn´t  allow the user to override the System Date (meaning current date here.), so when filing an order you can´t save a Purchase Order as though it was in a previous date (specified by you).

=================================================

It seemed like a little "cheating", there are programs that intercept the PC kernell, return the "current date" and replace it with the date you specify. 

But then, programmers know better!  Shareware creators are able to detect if there was some change in the date, and then  they end the trial period immediately...