Provide an ambient visual cue on how old a comment is. First idea is to add a subtle color tint to the background of each comment, that goes by the logarithm of the comment's age from reddish ("hot", written in the last couple of hours) to bluish ("cold", written several months or more ago).

Old threads occasionally get new comments and get readers in via them, and the date strings in the comments require some conscious parsing compared to being able to tell between "quite recent" and "very old" comments in the same thread by glance.