Lark automatically calculates your time zone based on your phone's time. If you look at the graph, you will see a duplicate day for the day that you changed time zones. Don't worry! Lark is still tracking your activity and will display it as two time zones.
To see your activity for the day that you changed time zones, tap on the activity for one of the time zones. Both time zones link to this same list view and are broken up by time zone.