the iproperty you're looking for is not even me: the only thing that comes to mind is that you make a custom that you update by hand or with an ilogic rule (datestring and timestring return current date and time), but also this I don't see how to connect it quickly to the press as there is no event triggers (if you're okay you can update it at any opening of the document, but so you wouldn't know the time).
a little more sophisticated macro would solve the problem but it is linked to a lot of "personal" factors of your workplace.
try to help, if not really help.. .
Bye!