I need to show app usage time in my app

Can't you measure it directly ?

• start when app comes to foreground
• stop when it goes to background
• save to userDefaults just befor

There is some sample code for how to use a DeviceActivityReport within your app's view hierarchy here: https://developer.apple.com/documentation/deviceactivity/deviceactivityreport

As for how to implement an extension that renders the report's UI based on usage data, if you select File > New > Target... > iOS > Device Activity Report Extension in Xcode, it will create a simple extension that you can then modify to fit your app's use case.

If this is not sufficient, please file a request for supplemental sample code using Feedback Assistant.