B2C Commerce timezone setting can only be updated within the first 60 days of your go-live date. At the end of the 60-day period, the option to change the timezone setting will no longer be available in your Reports & Dashboard instance.
Timezone setting can be for a specific region or a specific GMT timezone. If you change the timezone, existing order data is updated accordingly. For traffic data, the timezone only changes for reports generated after the timezone change.