The timezone is UTC.
Almost all network services will use UTC for any time values used in the protocol. It's the industry standard.
3.4. Exchanging votes
Authorities divide time into Intervals. Authority administrators SHOULD
try to all pick the same interval length, and SHOULD pick intervals that
are commonly used divisions of time (e.g., 5 minutes, 15 minutes, 30
minutes, 60 minutes, 90 minutes). Voting intervals SHOULD be chosen to
divide evenly into a 24-hour day.
Authorities SHOULD act according to interval and delays in the
latest consensus. Lacking a latest consensus, they SHOULD default to a
30-minute Interval, a 5 minute VotingDelay, and a 5 minute DistDelay.
Authorities MUST take pains to ensure that their clocks remain accurate
within a few seconds. (Running NTP is usually sufficient.)
The first voting period of each day begins at 00:00 (midnight) UTC. If
the last period of the day would be truncated by one-half or more, it is
merged with the second-to-last period.