What is the time zone used for tweets in Twitter API?
The created_at field for a tweet in the Twitter REST API gives the timestamp in UTC.
If a user has configured their timezone with Twitter, the utc_offset field gives you the difference in seconds between their timezone and UTC. So,
"utc_offset":-28800
corresponds to -28800/3600 = -8 hours from UTC.
There are a couple of suggestions that this doesn't get adjusted for daylight savings.