My browsers are set to purge all cookies upon exit. And I typically exit at least daily.
Then you log in to HN again, which enables HN to tie your new cookie to your old cookies and tell all its advertising and tracking partners.
HN has no such partners, of course. But many other sites do, and correlate cookies in order to tie devices together.
Invalidation is hard. Once you accept you have to tread down the road of accepting and deleting cookies and local storage I see no reason to wait 30 days. Why not purge them right on (tab)closing?. If you really care about privacy that is. Which could have been set to be default on the next update btw.