Screen Aware is an app that helps you manage your screen time. It displays an overlay timer stays visible above all other apps that acts as a constant reminder of how long you have been using your phone.
The app can automatically detect whether you are actively using your phone or not. It will only increment the timer when you unlock your phone and will pause itself when you switch off / lock your phone.
It also provides app usage statistics, showing how much time you have spent on each individual app.