Exclude timestamp when merging

Hi, I have a question. Is there any option to merge two books in different formats (example: epub and mobi), so i could merge all metadata except timestamp (added to library date)?
Example: i have one .mobi ebook with nice metadata added some time ago, and some time later i add same book just in different format (.epub, wrong metadata), but when i marge them it copies everything, even the date when i added the first book.
Then I want to save all updated books to my smartphone, but i need to search them manually, because dates for the merged books are wrong, which is a little problematic ;)
Maybe there is an option to exclude parts of automerge? Or a plugin?

Thank you for any help :)