Thanks for the info! Didn't think about "creation date", though it's of no use to me right now because I've copied my entire library after Calibre created quite a lot of converted formats (beginner's "mistake"). I can still try to identify those manually, though.
A custom column wouldn't help I think. Metadata is assigned to a book, not a single format, isn't it? If there is a way to assign metadata to a format instead of a book I didn't find it yet, and I've looked everywhere (Google, forum search, Calibre manual, etc.).
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