The Time Stamp Counter on modern CPUs runs at the same speed regardless of the CPU frequency or idle state, providing that the base clock isn't modified. Under

The Time Stamp Counter on modern CPUs runs at the same speed regardless of the CPU frequency or idle state, providing that the base clock isn't modified. Under normal use, that's absolutely fine - the OS can calibrate its internal clock against a wall clock timesource at boot time, and then use the TSC (which is very cheap to read, unlike the actual RTC) as a reliable time source. The problem here is (apparently) that calls that used to cause Windows to read the RTC now give you a TSC-based time instead. If the TSC speed has changed since boot (which should only happen if the user has explicitly changed the CPU base clock, never under normal use) then you have no way to calibrate the OSes idea of time against an actual time source. This is a problem if you're trying to perform accurate benchmarks when under/overclocking. So yeah, Windows 8 appears to use the CPU TSC without recalibrating when there's a change in TSC tick rate, but that's because the TSC tick rate isn't supposed to change.