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Calibre's "Modified/last_modified" column is the date and time a book's metadata was last changed. A book's metadata is stored in the library database, which is the metadata.db file in the library folder.

A file system's "Date modified" is the date and time the content of a file was changed.

Format files (book content), such as .epub, .mobi, .pdf, .cbz etc, are not changed when the metadata changes.

Because it is written when the metadata is changed, the file system's "Date Modified" of a book's 'metadata.opf' file will be normally be equal to or close to Calibre's "Modified/last_modified" column.

The Polish Book or Embed Metadata tools can be used to inject the current metadata values into a book's format file. When used, the file system's "Date modified" of a book's format file will be updated to the date and time the Polish or Embed operation was performed.

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