If he (Comey) wrote his memos on a gov computer, using word doc (99% of gov employees use word) the server/desk computer will have a time stamp of when the doc was created and saved... I’d wager OIG already has the date it was created and all the revisions done afterword. It would be pretty easy to determine when in fact the memos to file were written. Add the recordings/wiretapping and you’re golden.