NtQueryPerformanceCounter - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers 
#ifndef _NTEXAPI_H
#if (PHNT_MODE != PHNT_MODE_KERNEL)

//
// Performance Counter
//

NTSYSCALLAPI
NTSTATUS
NTAPI
NtQueryPerformanceCounter(
    _Out_ PLARGE_INTEGER PerformanceCounter,
    _Out_opt_ PLARGE_INTEGER PerformanceFrequency
    );

#endif
#endif

View code on GitHub
#ifndef _NTZWAPI_H

NTSYSCALLAPI
NTSTATUS
NTAPI
ZwQueryPerformanceCounter(
    _Out_ PLARGE_INTEGER PerformanceCounter,
    _Out_opt_ PLARGE_INTEGER PerformanceFrequency
    );

#endif

View code on GitHub

This function is documented in Windows SDK.

PerformanceCounter

Result is number of processor ticks after last reset.

PerformanceFrequency

It's number of processor ticks per one second.

Another method of uptime calculation:

UpTime = PerformanceCounter / PerformanceFrequency;

Related Win32 API

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