There is no way to display in the library view the date that a format (epub, mobi, etc) was modified. There might be a plugin that gets the data, but if so I haven't run across it.
The modified column shows the date that the metadata was last changed. When the column was added (fairly recently), the date was set to 1/Jan/2000 for all the books in the library. Any book changed since then will have a newer date.
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