In your settings > display post time in feeds > on. Should appear after the username. (:
We chose not to make it the default so that people are not discouraged from commenting on old posts. On Hubski it is not unusual for discussions to go in for days, weeks and even months.
That makes sense. For myself, I'll turn on time stamps, because a lot of the content I follow is time sensitive. Such as new technology and academic advancements. It is probably worth making the option more visible somehow. Such as putting it in the faq or somewhere in the website introduction / tutorial.
If you comment on an older post, it gets reinjected into the feed. While it is useful for breaking news or the latest tech, I would recommend you comment on other older posts that aren't time sensitive. I always love it when I see an old post come back to life. In fact, I also love it when old "latest tech" posts come back to life as it often gives an interesting perspective: