Thanks, now I recall and see what they are doing. Unfortunately, it's not tracking read posts at all as I got a number of threads that were marked as new because they had new posts. Clicking on the last new post should have shown me the whole thread as I'd never read it. Instead, it brought me to the first new post since my last visit.
That's ok and it's surely easier. You right, we could just set a cookie to do a similar thing. Would that be sufficient for you even though it's not truly correct?
Rod, thanks for your attention. Well, the AVS forum does not work perfectly if you don't visit it in a long while (definition of long while is like greater than a few weeks), but if I visit it daily, I am assured that the bold threads contain unread posts, and that the little icon to the left of the bold thread title takes me to the first unread post. 'First unread post' works out to be the first new post since my last visit FOR threads that I have previously opened AND (I'm not 100% sure of this next part) ALSO gotten to the last page of that thread. But if I have not opened a thread, log out, log back in next week, then that thread's 'Go to first unread post' will take me to the correct unread post for me. I think they might be using anchors not sure, as I seem to recall getting taken to the correct page of the thread, with the new post near the top of the window. So perhaps there is a cookie that they read showing which threads I have read in the last week and calculating the right anchor to take me on the right page.
I probably have rambled on with some errors above. If you were able to bold threads with unread new posts, and then somehow mark the unread posts with some icon, that would be the optimal solution. Any reply later in time than your cookie indicates would bold up the parent thread title, and add an icon to that reply + all replies later in time. Does that sound correct-ish?