If your business has "Online/Phone" turned on as a location, customers will be able to see times on the booking page in their own time zone. This also means that any booking they make will appear to the customer in their own time zone, and to you, they will appear in your time zone.
Editing customers time zone
You can edit the time zone for every customer under "Customers". Just select the customer and use the drop-down next to the "Timezone" option.
Booking an appointment for a customer in a different time zone
To book an appointment for a customer that is based in a different time zone, you will first need to create the customer record. For this, you can use the "+ Add Customer" button.
Now you can use the drop-down menu next to "Timezone" to edit the customers default time zone, and then use the "Book" button, to book the appointment for the customer.
When entering the time on the booking form, make sure to enter it in your timezone (as if the appointment would be in your time zone). The customer will still receive reminders showing the times in their own time zone.