For the app to update, it needs to be “woken up”. This will happen when you manually open the app. If you have location services enabled, it will also update when you make a “significant location change”, as determined by iOS.When the app is woken up, it will add all of the usage since the last time it was awake to the current hour. As long as you open My Data Manager before turning off your phone, the usage total will be accurate.
Customer support service by UserEcho