[rebl]

Thank you all, I'll keep all these in mind. In the end, it seems that the default Modified column seems to work, I am still testing but until now it's all right.
The "modified" value is changed if I double click (or F2) a cell, even if not modifying anything, and that is still a little unsatisfying, but on the other hand if I press E to edit indivually a book's metadata and then I decide to cancel and not save anything, the "modified" remains unchanged.

The current "tweaks" I'm using are the ones below, and they work all right
gui_timestamp_display_format = 'yyyy-MM-dd, hh:mm:ss'
gui_last_modified_display_format = 'yyyy-MM-dd, hh:mm:ss'

Very interesting, not any cell produces a change in "modified" time if entering cell with F2 and leaving with no data enter/change.
I have Genre cell which is defied as hyerarhical - and this one triggers a change in 'modified' even if I only enter and leave the cell.
I have antoher custom, simple text cell, #rg and this one does not trigger a change in "modified" if I don't modify it, just F2 then leave.