Android: Screen Stopwatch
Screen Stopwatch tells you how long you spend on your phone each day. Each time you unlock your phone, the stopwatch continues to count. Screen Stopwatch is part of Digital Wellbeing Experiments, a platform to share ideas and tools that help people find a better balance with technology.
You can download the app from https://play.google.com/store/apps/details?id=com.digitalwellbeingexperiments.screenstopwatch