This is a feature of any device with a battery-backed real time clock (RTC). All x86 machines have this feature. Pretty much anything with a clock that doesn'

This is a feature of any device with a battery-backed real time clock (RTC). All x86 machines have this feature. Pretty much anything with a clock that doesn't need resetting every time you turn it on can do this. An RTC chip is just a super low-power counter, and it costs almost nothing to toss in a wake-time comparator, so all of them do. When you set the wake-time alarm the host CPU really does power down 100% -- right after it sets the RTC chip's wake-time registers. This is a long, long, long way from a microcontroller. It's not Turing-complete and has only a few dozen bits of storage for the counter. They're also manufactured on incredibly ancient fabrication processes (like 350nm until very recently) for lowest possible leakage.