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@boudicca - the 'timestamp/Date' is set to the system timestamp when the book is first added to the database, after which it isn't changed by calibre internally. However it is user modifiable. Some users re-purpose it to be the date they finished reading the book. Some re-purpose it to be the date they purchased the book, which may be before or after they added the 'book' record to the database.
It is almost certain that you didn't 'lose' calibre's last_modified date in your books, you did something (singular) that rippled (propagated) a metadata change through multiple (all) books. It will happen again.
BR
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