For those searching for the Peace of Mind Apps: It seems it’s been depreciated for an existing function in Android 5.1:
The amount of time you’ve had “peace of mind” (based on Android’s “no interruptions” setting).
Source: Blog - Software features of the Fairphone 2 - paragraph Interactive Clock
The function can be found at: Settings > Sounds & Notifications > Interruptions
About how to reset the counter: No idea, but maybe the counter is in fact buggy.