RtlTimeToSecondsSince1970 - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTRTL_H

NTSYSAPI
BOOLEAN
NTAPI
RtlTimeToSecondsSince1970(
    _In_ PLARGE_INTEGER Time,
    _Out_ PULONG ElapsedSeconds
    );

#endif

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This function is documented in Windows Driver Kit.