For as long as I remember, chat timestamps haven't stated which day is "today" accurately for me. It is probably noticeable to me because of my time zone — I live on the east coast of Australia (UTC+10).
I just posted in Adult Main Channel, where the last post was yesterday 6am (by my clock), and the timestamps look like this:
Yesterday at 02:36 PM · (<- posted two days ago, but chat considers this "yesterday")
01:28 AM · (<- the earliest post from yesterday, apparently chat considers this "today")
(... a few dozen messages skipped ...)
06:07 AM · (<- the last message from yesterday)
Wednesday at 08:40 AM · (<- my message from today)
Really it should say something like:
Monday at 02:36 PM ·
Yesterday at 01:28 AM ·
Yesterday at 06:07 AM ·
08:40 AM ·
My best guess is it's going by server time or UTC+0 to determine what the current date is, but it should really just go by my PC clock if it's using that to display timestamps anyway!
It's a bit past 11am here now (so, past 1am UTC) and the timestamps have turned into:
Monday at 02:36 PM ·
01:28 AM ·
06:07 AM ·
Today at 08:40 AM ·
So it acknowledges today as today finally, but the labels are blank for yesterday rather than blank for today (with yesterday labelled). Unless there's some other factor there, like number of messages on either day?