badbob001 - Hover over format in Metadata Edit, viz:
But for your purpose, comparing dates, it's probably easier to use your file manager - select the book (i.e. highlight in book list) press 'o' and sort file manager window (explorer, finder, nautilus... whatever) on Date Modified.
But as theducks said things like Polish, Embed, Modify plugin etc will change the metadata in some file types but not the content - which is I assume is what interests you.
Generally speaking any change, to any file, by any program, will change the Date Modified.
How I manage originals: After using the downloaded file to create the book in calibre, I rename it to *.*.orig and add that file to the book. Then at least I can find it. Periodically (random intervals) I'll cull .orig files from older books that don't have '_keeporig' in #admintags.
If I want to retain a record of the original file name, source location (most of my books are not commercial publications) etc, I put the data into the #booknotes column (comments like).
BR