I just noticed that the times on comments did not square off with the other info on the site. I took a screen capture (for a different reason), but the times in the comments showed that "tchrist" made a comment "4 mins ago", while his profile stated that he was "last seen 6 mins ago". Is this a bug or is there a general guideline stating that these times should not be relied on to construct any kind of precise timeline?
According to Shog9, "last seen" is only updated every 15 minutes and thus is not as accurate as other timestamps (e.g. post and comment timestamps):
The "last seen" date is only updated every n minutes, where n was 15 the last time I bothered to check.
So when this time is recorded, it won't be updated again for at least another 15 minutes, no matter how many times the user accesses the site during this period.
The last seen is inaccurate intentionally, as mentioned in Laurel's answer.
The timestamps on comments, questions, answers are precise. For recently posted ones, the time will be shown as x mins/secs/hours ago, as seen in your screenshot, and when you hold your mouse pointer over it, you'll see the UTC timestamp: 2017-10-20 21:44:42Z.