More than once it happened to me that I found I could improve my comment or wanted to add something, reopened it at a timestamp of "3 minutes ago", and tried to save the edit only to get notified that editing is only possible within the first five minutes after posting, whereas after cancelling the edit the timestamp read "4 minutes ago". This is somehow embarrassing because a user is lead to the wrong conclusions as to the time left to alter one's comment. But as Wrzlprmft suggested, this might be a bug.
Wouldn't it be a better solution to enable opening the comment within the exact timeframe, allowing to exceed it eventually by some extra time when the user wants to save it even after those 5 minutes have gone by (perhaps with a warning that the edit will be rejected after ...minutes), and refusing access to editing exactly after five minutes?