The time zone is a global setting and you can only have one time zone. If you are sending reminders to multiple time zones then you should set the time zone to your main location but then set the “Reminder Schedule” appropriately.
Example: If you are located in Eastern Time and sending reminders all across the country including Pacific Time, you may want to set the start time to something like 11 AM instead of 8 AM. This way, customers in Eastern Time will get their reminders starting at 11 AM Eastern and customers in Pacific Time will get their reminders starting at 8 AM Pacific Time.