Checked the Properties, tried to setup "Compatibility with Windows 7" and also "Run as Administrator", but no change, date is still "Unknown", so reset both to defaults
Is there any other "Property" to v erify?

it looks OK, see attached screenshots:

in windows Explorer


when using "Open" in Gimp


it seems to be the Gimp "File->Open" function which is in trouble

OMG, looks like you have an infection!!!
Just kidding...

performed a complete scan of this PC: no malware
still the same problem
I guess I will reinstall GIMP

reinstalled, but no change
even removed (renamed) my User/.gimp-2.8 folder, to access a "clean" Gimp without any additional plug-in/script: no success, that column always show "Unknown"
I will give up...

@Diego:
I notice you've got your system set to Italian, but Gimp set to English.
Try setting Gimp to Italian just in case there is a problem with date/time formatting.

no success again...thanks anyhow

I'm curious whether some guru is able to "read" inside the module used by Gimp to browse the folders in order to perform the "open" function, to understand what kind of dates it expects (Jonathan, I did also the opposite try, following a suggestion of a dear friend of mine: set system region to English(US) and Gimp the same (either explicitely to use English(US) language or using the system setup modified to English(US): no change!).
Wouldn't it be possible for Gimp (like for any other image processing SW) to use the Windows Explorer (as an option)?
Has this problem maybe to do with 64bits or Windows 8.1? (at home I have 32bits and Windows 7, and it has no problems)

The "Open Image" dialog in Gimp is basically just a standard GTK file-chooser dialog.
Have you tried opening files in any other GTK-based applications such as InkScape ? If not, perhaps you could do so.
It could be a compatibility problem between GTK and Windows 8.

(And no, I don't think it would not really be possible for Gimp to use the Windows Explorer; those applications which do use Window Explorer to manage files are generally Windows-specific; Gimp is a cross-platform app and relies upon the lower-level software such as GTK to adapt to the specific platform being used; getting it to use the Windows Explorer dialog when running on Windows platforms would require a lot of work and goes against the grain of cross-platform policy.)

Jonathan,
I downloaded and installed (using std options) Inkscape
Launched and asked to open one of my images folder: no problem, as you can see below


maybe, Inkscape uses the std Windows explorer

@Wallace
"Are you asking how to add Edit with Gimp to the right click menu in windows explorer?"
no, thanks, Wallace, I do not have problems in OPENING an image with Gimp, it's just that the folder does not show in the list the "date last updated", so that I do not know when did I update each image

Yes, that is the Windows dialog. (InkScape uses the GTK dialog on Linux.)
Perhaps you could try some other GTK-based apps. See http://gtk-apps.org/

What I'm trying to get at is whether this is a Gimp problem or a GTK/Windows problem . (Or perhaps just your problem! )

Jonathan, I guess that something went wrong during the initial OS installation, performed by the seller of this laptop.
But instead of redoing now a clean installation of Windows 8.1, I wait upon the incoming (one or two months) Windows 10 and then do a clean install.
So I will "survive" without that date for now...

@ all those friends who did try to help me in this "battle"
without waiting for a new clean installation when windows 10 will come out, I decided to create a new user, and now can see the dates of the folders and files in the open module! eureka!
I don't know what was (is) wrong in the existing first user (who still continues -if logged in- to show "unknown" in the dates, but with the new one everything is OK: I can use Gimp in English as well as in Italian; just to "complete" the info, the open module (which is a gtk module as Jonathan said) always works in Italian (the OS installed language), even when Gimp displays menu and other info in English.
OK, thanks for your patience in supporting my "claims".
I also know that what I say cannot be of much help to others, because I didn't understand the "why", but at least this could be said: the creation of a new user can solve such kind of problems without reinstalling the whole system from scratch.

raises it's ugly head again in Windows 10. gimp 2.10.36