I'm not sure how different devices implement it, but the security chip can simply count the time it was powered on, it doesn't have to rely on wall clock time.

I'm not sure how different devices implement it, but the security chip can simply count the time it was powered on, it doesn't have to rely on wall clock time. (Relying on wall clock time caused a bug in an early iOS version of this feature, where it would show a really long delay when the clock was reset, and there was no way to set the clock correctly)