Here's all you need to know about what's available and what's not in Splunk Free :
As you can see you don't have access to modify any user/role configuration:
All accesses are treated as equivalent to the admin user. There is only one role (admin), and it is not configurable. You cannot add more roles or create user accounts.
In case you purchase Splunk you can change the timezone configuration as follows :
Hope that helps.