Before we started doing it this way, the threads would fill up with far more off-topic comments ("why is this 3 day old post on the frontpage?" and so on). That was a much bigger problem. Relativizing the timestamps to reflect the re-up time was our attempt to address that. Overall, it has worked ok—that is, while it does still lead to offtopic confusion and complaints in the threads (boo) there is far less of it than there was before (yay).

I'm open to suggestions of how to do it better! But you also need to consider the cost of adding explicit details to the UI. If we did that every time something like this came up, HN would have become an unreadable mess a long time ago.