Small bug/feature request: The "relativetime" class is not used in comment timestamps.
While I was writing this Greasemonkey script, I noticed that the relative timestamps in the comments are not tagged with the "relativetime" class, the way relative timestamps are everywhere else on the site (i.e. questions, answers, user profile, question lists).
Edit: Found one more place where the class isn't being used: on the message for a bounty, stating "this question has a bounty of X expiring in Y days". Also, in that case the timestamp tooltip is in ambiguous m/d/yyyy format, which could be interpreted by some users as d/m/yyyy. The yyyy-mm-dd format would be more consistent with the rest of the site.