While using the mobile application (Android or iOS), there is no need to establish your time zone. This information is pulled from your phone's physical location and events will show dates/time based on that said location (when viewing within the mobile application).
There may be unique instances where you desire your time zone within the web app (desktop or laptop) set at something different than your current location. For example, you may have your time zone set as Eastern Standard because you live in Detroit. If you were to then drive to Chicago (which is Central Standard) and opened the mobile app, the time would be displayed accordingly (in Central Standard). Your selected time zone would still be displayed and established on the web application but your phone will update based on your current location.
You have control over the time zone in the web app but when using the mobile app, it will always display based on your mobile's location.