[rebl]

Thank you all for your replies,
@BetterRed, the tweak you recommended,
gui_last_modified_display_format = 'yyyy MM dd hh mm ss'
did work, but unfortunately any double-click on a cell (entering edit mode), even without modifying anything, actually refreshes the "modified" field, so indeed that's not very useful.

I still haven't managed to make the {last_modified:format_date(iso)} work. I had made a custom column with this value (as seen in the attached picture) but after entering edit mode in a cell or entering metadata edit mode for a book (individually) my custom colum is filled with "BAD DATE" value.
Maybe the syntax {last_modified:format_date(iso)} is not correct?
On the other hand, if {last_modified:format_date(iso)} produces the same output as the "Modified" column then ther's no point using it.

I couldn't find in the documentation the difference between timestamp and modified, and what exactly is gui_timestamp_display_format used for.

P.S. That's a bummer, after restarting calibre all the values in the "Modified" column have been resetted to exactly the same minute and second, losing my previous differences between records.

Maybe what I'm looking for is timestamp and not "modified" but how do I use it?

P.P.S. I just made a column (with value "built from other columns") {timestamp} and it shows different values for seconds, but not the metadata modified date. Apparently timestamp show the moment the book was added to the library (?). Please correct me if I'm wrong.
So there is no useful function to just see when was the metadata last modified (maybe the only way is using "Everything Search" from "outside"?)