The time zone that a Twitter user selected in their Twitter Settings.
Note that Twitter uses city names to specify timezones. For example, using this target, your filter might match against Edinburgh and against London, but these both represent a single timezone.
1. Filter for Tweets from the same timezone as Tokyo:
2. Filter for Tweets from users who have not selected London as their time zone:
Here's a complete list of the values you can filter against with twitter.user.time_zone: