I do not believe there is a feature like this built-in. The cloud portal does log when the app is restarted plus other critical alerts, but they are not aggregated in a way that is easily reportable.
You might consider using a tool like https://uptimerobot.com/. I haven't done this personally. My only caveat right now is that you get a downtime page when the app is offline, and you'll want to make sure the robot treats that response as actual downtime.