Most of the time the time stamps shown on messages in chat show the time to me in my local time. The fourth post in this large image is mine and that's what time it was here.
But the previous two messages use yst. What is that? "your standard time"?
...cropped montage of this
...using tooltip
It's a bit easier to see if you look at a chat that has a bit more activity than the one in your example but we use a three-letter abbreviation for chat messages up to a week old.
So, something posted today just has the time, while something the prior day says "yst" for yesterday. Further out, we switch to days of the week, so "Sun" and "Sat" for Sunday and Saturday.
After you get to a week ago, you get to the full date with a three-letter month indication.
If it were some sort of time zone indication, it'd (probably) be in all caps. As you can see from the "Sun" casing, we do follow those casing rules.
It means "yesterday". You can see a hint in the text above the last message: "1 day later".
Chat timestamps need to be compact, so it's not spelled out.
st that's throwing me off. (e.g. est, dst) There's probably room for something like ystrd but I don't think I'm going to make that my first feature request. Thanks!
To expand on the other answers, it indicates "yesterday" that, rather confusingly, is the period of time between 24h and 48h ago, regardless of the time of the day. So, for example, "Jan 15, 17:00" will be "yesterday" if visualized on "Jan 17, 16:00".
Note that this is not affected by your timezone.
