In most circumstances, once you've used Thrive for awhile, Thrive's default time zone should not be changed, so please choose the default time zone carefully.
This is because Thrive caches a lot of data so most of your reports are near instantaneous, no matter the date range. And all of this data is cached based on your default time zone.
If you do need to make a change though, once you make the change, all future stats will start to be aggregated in the new time zone you sent, which means your past data will be off, since they will still be according to the old time zone (i.e. it will be the same data by now marked as a different time zone).
For self-hosted users, if you wanted to change your default time zone now, it is possible to do it through the Thrive database directly in the table system_settings.
If this is a new installation, then this is not a problem, since you won't have much data in it anyway.
If it's an existing Cloud installation and you still want to move forward with changing the time zone, please submit a support ticket.