For example, if you want to create a clock app for AppleTV, you can't trust device's clock (NSDate) at all.
Here is the story.
If you leave AppleTV unplugged, AppleTV's clock does not proceed. If you plug-in to the power, it often fails to get correct time from NTP server.
In my case, AppleTV does not even try to connect NTP server again for more than 30 minutes.
If I reconnect the wifi, sometimes AppleTV does fix its time, but often does not.
If I unplug and plug-in again, it often fixes the clock.
This seems to be an OS bug to me.
By the way, you can check AppleTV's clock by seeing Xcode's Device console, or just ask the Siri Remote.