Microsoft KB Archive/239818

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DOC: Obtaining System Time Using KeQuerySystemTime

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Q239818

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The information in this article applies to:


 * Microsoft Windows NT 4.0 Driver Development Kit (DDK)

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SUMMARY
The Kernel-Mode Drivers Reference included with the Microsoft Windows NT DDK 4.0, Part 1: Kernel-Mode Support Routines, Chapter 5: Kernel Routines, Section KeQuerySystemTime, states the following:

KeQuerySystemTime obtains the current system time.

'''Parameters

'CurrentTime''

Points to the current time on return from KeQuerySystemTime.

Comments

System time is a count of 100-nanosecond intervals since January 1, 1601.

This function obtains the current system time maintained by Windows NT. The system clock holds the system time in units of 100-nanoseconds, but has an update period set by the system timer interrupt. This is typically around once every 10-milliseconds. In a Uniprocessor HAL, an increment value of approximately 100,000 (100000 x 100-nanoseconds = 10 milliseconds) is added to the system time for every timer interrupt.

MORE INFORMATION
The exact increment value can be obtained by using the KeQueryTimeIncrement function.