There are some tools in the JobSpy PI that wrangle the Modified Date.
And there's a
Last Modified plugin from capink
That's as much as I know about them.
When I realised how calibre's Modified Date was used I decided to ignore its existence. In one of my libraries I have a Last Changed Date column, I edit metadata in the book list which means its easy to tab into that column and hit '=' when I deem it worthwhile.
BR