shims: correct _dispatch_get_nanoseconds on Windows

The NT time representation uses 1/1/1601 as epoch as opposed to Unix
which uses 1/1/1970.  Account for the offset in the calculation.  This
corrects the calculation of now.

Author: compnerd

src/shims/time.h

@@ -108,13 +108,14 @@ _dispatch_get_nanoseconds(void)
 	dispatch_assume_zero(clock_gettime(CLOCK_REALTIME, &ts));
 	return _dispatch_timespec_to_nano(ts);
 #elif defined(_WIN32)
+	static const uint64_t kNTToUNIXBiasAdjustment = 11644473600 * NSEC_PER_SEC;
 	// FILETIME is 100-nanosecond intervals since January 1, 1601 (UTC).
 	FILETIME ft;
 	ULARGE_INTEGER li;
 	GetSystemTimeAsFileTime(&ft);
 	li.LowPart = ft.dwLowDateTime;
 	li.HighPart = ft.dwHighDateTime;
-	return li.QuadPart * 100ull;
+	return li.QuadPart * 100ull - kNTToUNIXBiasAdjustment;
 #else
 	struct timeval tv;
 	dispatch_assert_zero(gettimeofday(&tv, NULL));