This read-only PerfCounter64 reports the elapsed monotonic time for the current interval in milliseconds, which per its description can be less or more than the nominal measurement interval depending on whether the time-of-day clock was moved forward or backward during it. An administrator checks this when an interval's statistics look off, since a clock adjustment (like an NTP correction) mid-interval can make the elapsed monotonic time diverge from the expected interval length. For example, if f3EsaProbeStatsSoamPmExtElapsedTime for an interval is noticeably shorter than the configured measurement period, an operator would suspect the system clock was stepped forward during that interval rather than assuming a measurement error.