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.
Note that a precise value for "last seen" is no longer shown on other users' profiles (except as a moderator). Anyone who just visited the site will be shown as "last seen this week". (The network profile will show a little bit more detail — "last seen today".) It's also vague with other timeframes. For example, looking at OP's meta profile, I see a specific date in 2019 since I am a moderator. But when I am logged out, I see "Last seen more than 3 years ago".
The timestamps shown elsewhere (on questions, answers, comments, etc.) are accurate, though rounded. The timeline of a post (under the vote icons) will show you the exact timestamp, especially if you hit "toggle format". You can also hover over various rounded dates to get this information.
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.