The timestamps everywhere in TopAnswers appear to respect the system locale of the user's OS, thus showing as local time. For this reason probably (my guess), they never show a time zone marker. Without a time zone, as well as without an explicit note anywhere on a page saying which time zone the timestamps are, it is quite natural to assume they are in your local time zone. The assumption is all the more strengthened as you find it confirmed in the course of using the site, for instance by observing timestamps during chatting.
I may have found one place where timestamps shown are most likely not local time and yet they have no time zone marker about them either. I am talking about Post History pages. Here is one example from Meta:
The hottip circled is shown for the `(edited, 2 months ago)` label above it. The time shown is 20:20 (or 8:20pm). When I look into the History, however, I see this:
The edit time there is 17:20 (5:20pm). The three-hour difference matches the current offset for the time zone I am in, EEST (Eastern European Summer Time).
The issue appears to be network-wide, as I have observed it at dba.ta as well. As a crew member, I get post flag notifications, and today I received this one:
The question's History page, however, shows this:
So I get 8:56 in the notification, but the history shows the time as 5:56. We have the same three-hour difference again.
My understanding, therefore, is that History shows the timestamps in UTC.
I believe it would be best to show all timestamps everywhere uniformly, so Post History should respect the user's locale as well. However, my primary concern is for the timestamps not to be confusing. Therefore, I would be as happy if they just had a 'Z' attached to them to indicate that they are indeed in UTC.