As of v3.0.23 reports run in the time zone set on the browser. See Common Report Parameters for details.
Previous Behaviour
Previously reports ran in UTC. This means that start and end days were based on midnight UTC and all dates and times were displayed in UTC. Be aware that after the upgrade reports will run in your browser's time zone and so will not be directly comparable with previous reports as the time periods will have shifted by a few hours depending on your time zone.
Scheduled Reports
Scheduled reports already had a time zone specified for the time to run and this is used for the time zone when pre-existing scheduled reports run from 3.0.24 onwards. Note however that the report itself will now run for that timezone and not UTC which was the previous behaviour. For example, if a report was scheduled to run at 6 AM AEST (UTC +10), after the upgrade the report will run for the AEST (UTC +10) time zone.