Open Bookings > Settings > Availability. Full steps below.
Where: Company time zone: Settings > Company Profile > Time zone. Booking time zone: Bookings > Settings > Availability > Time zone.
Plans: All plans for the company time zone. Booking time zone: plans that include Bookings.
Steps
Open Bookings > Settings > Availability.
Pick a zone in Time zone, or turn on Use company time zone to keep it matched.
Click Save. Working hours and customer time slots use this zone.
Rules and limits
Changing a zone does not move existing bookings; they keep their real moment in time and are just shown in the new zone.
If Google Calendar itself is set to another zone, Google shows the same moment converted to that zone.
Still stuck?
If a step does not match what you see on screen, stop there and open a ticket from Support > My Tickets with a screenshot of that step. The team will look at the account directly.