Indeed - there are some fundamental assumptions of a 86400 second-long day. We don't trust the browser to do anything other than keeping track of how much time has passed; we don't use the time zone in it as it may not be your server-selected TZ.I think the labelling of the posts as 'Yesterday' comes about because the day effectively has 25 hours; 24 "normal" hours, plus the 1 hour of time reversal.
My understanding is that the roll-back occurs at 1am (so that it rolls back in the same day - as opposed to rolling back into the previous day/date).Anyway... I created that at 12:00 AM this morning before DST at 2:00 AM, and even when coming on again a while ago, it stayed as 12:00 AM and didn't go back one hour to yesterday at 11:00 PM.
I think the problem actually lasts all day for the day after the clock change with posting times being an hour out. It isn't very apparent because Xenforo list things as "a moment ago" or "55 minutes ago" etc, so you don't see it for recent posts and the timestamp for older posts is less relevant. Of course the "Yesterday" issue which makes it more apparent only exists for an hour.The issue mostly comes up for 1 hour of 2 days of the year (though there may be an edge case where it comes up relating truly "yesterday" stamps). The behavior is also going to be slightly different when going forward vs back.