Two problems. First, you may need to sort both files before you do this. If the order of entries might change between days, then "diff" isn't a great solution.
Second, you'll probably end up printing severals lines as being both added and deleted. "diff" isn't great at doing a set difference. It is looking for document edits and so can easily report a big chunk of the "bigger" file as being changed and then show the subset of that chunk that was already there in the "smaller file" (and didn't change).
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tye
(but my friends call me "Tye")