I look at the time stamps near each post (for example, your thread-opening post says "12 minutes ago") to tell when they are posted. I have been on Hacker News for 1113 days now, so I've developed varying strategies for reading threads over time. For some of my favorite authors of Hacker News posts, especially some of the persons on the leader board
http://news.ycombinator.com/leaders
who have high comment karma per comment, I look back to earlier threads by looking for their comments from their user pages, to find recent comments by those authors. Example: comments by mechanical_fish:
http://news.ycombinator.com/threads?id=mechanical_fish
Other times, I just skim comment threads to see if some new issue has arisen, or especially if a participant has linked to a reliable source, preferably a professionally edited source.