Despite the accepted answer that states all times are UTC on SE, it's clear that some times are relative and that chat time stamps reflect local time zones other than UTC.
Chat and iOS apps seem to localize time for me.
What parts of the Stack Exchange use time zone offsets that could be other than UTC to render times visible to end users and how is the Time Zone established for each such place?
With the exception of chat (which is a fact I just used today), all of the precise time stamps are based on UTC time.
The relative times that you see (such as "yesterday", "2 days ago", etc) are just that, relative based on 24 hour periods from the current moment, so there is no time zones taken into account. For example, a question asked 23 hours ago, regardless of date, is labeled as 23 hours ago. Where a question asked 25 hours ago is labeled as yesterday.
So chat is the only place where any time localization is occurring.
And based on Why does the Transcript and the actual chat room have different Timestamps?, the time localization only occurs in the user interface. Downloading transcripts shows the actual UTC timestamp.
Actually I was just able to confirm the "UTC Time" we see when opening the Achievements box (previously displayed when hovering our display name in the old top bar) is based on local machine time:
(screenshot was taken NOW, 16:01 UTC time and I just changed my local time)
