Sounds like a custom feature we would need to develop. So setting a date to go back and look at unread posts might be doable, but isn't something we could turn on "out of the box". The "cookie" resides on your computer, not ours and I have no idea how all of the mechanics of those things work.
This is something that would need to be custom (do-able but custom). Also, you would have to set that date everytime you come to the site since the next "last visit to LBG" is the "current visit to LBG".
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