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.