Terry Pinnell wrote:

BTW, with apologies for digressing, I just viewed it in IrfanView and
closed it again, and was surprised that did not change 'Date accessed'.
So then (without editing) I resaved it - but no change in 'Date last
saved'. Then I edited it and resaved, but STILL no change to 'Date last
saved'! FWIW, this was working on a backup copy on a 2 TB USB HD,
although I don't see how that's a factor?

If you talking about the meta-data in the image file, that won't change
unless the editor regenerates it. Many users have complained about
editing a photo in Irfanview, saving the changes, but the meta-data
still shows the same old information. In fact, part of the meta-data is
a thumbnail and users were frustrated that the edited photo did not show
an updated thumbnail in File/Windows Explorer. That's because those
show the meta-data's thumbnail instead of building their own.

I'd have to go research again but remember that users found an option in
Irfanview of whether or not to regenerate the meta-data when the image
file got edited. In fact, you and I had this discussion before (titled
"Thumbnail inconsistency") in the Windows 10 newsgroup about thumbnails
stored in the meta-data of an image file where I mentioned 'do NOT
enable "Keep original EXIF data" option' in Irfanview. If you want a
changed photo file to reflect the changes in its meta-data, you have to
tell Irfanview to generate new meta-data on the changed file that you
save; else, Irfanview propagates the old EXIF data in the changed file.