ScreenTime

モニタースクリーンオン（ディスプレイオン）時間

This App, developed in Tasker, measures your Screen-On Time. It starts counting time when the display gets turned on, and stops when the display gets turned off. It shows the time of the current "session" and the cumulatve time as permanent notification.

It does NOT reset the cumulative time when you recharge the battery (the Android Settings / Battery display does reset on charge complete, and that's why I developed this app).

In the app, you can select an icon for the notification, and you can set a time for how long you think the screen was on before. (On a just-bought device still on its first charge, use the time from battery usage / display.)

I use this on an Android(TM)-driven eReader to see how much time I spend reading.

No internet, no frills, and free.