How Does GetSystemTimeAdjustment Relate to System Clock Update Intervals?

Windows 7 Timing Functions - Understanding GetSystemTimeAdjustment Usage

Question 1: Time Increments and Synchronization

Yes, your assumption is correct. The GetSystemTimeAsFileTime function returns timestamps in increments that are a multiple of the interval retrieved by GetSystemTimeAdjustment. If no updates have occurred, no change should be observed in subsequent calls to GetSystemTimeAsFileTime.

Question 2: Discrepancies in Time Synchronization

The GetSystemTimeAdjustment function does not directly provide the system clock update interval. The interval is determined by the underlying hardware and is reported by the NtQueryTimerResolution function.

Typical hardware platforms return an ActualResolution of approximately 156,250 or 100,144 for NtQueryTimerResolution. This indicates that the system clock is updated every 15.625 ms or 10 ms, respectively.

However, the multimedia timer interface (timeBeginPeriod and timeEndPeriod) can override this resolution. Calling timeBeginPeriod with a smaller period (e.g., 1 ms) forces the system clock to update more frequently, even if the hardware does not support it.

In your case, the reported Time Adjustment of 0.0156 seconds matches the typical hardware resolution. However, the actual update interval may be different due to the multimedia timer being set to a smaller period (e.g., 1 ms).

Implementation Notes:

To accurately measure the system clock update interval, use NtQueryTimerResolution. Be aware that calls to timeBeginPeriod/timeEndPeriod can affect the system clock, and frequent calls should be avoided.

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