I have version 1.4 of Calibre, and the last_modified date will be changed if the edit metadata box is displayed for a book and no change is made. I just happened to be browsing through some books that I had added last year, and the last_modified date was changed when I went into edit metadata for the book. I didn't make any changes, but the date was still changed.
It looks like as soon as you use edit metadata for a book, the last_modified date will be changed whether you make a change or not.
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