On Windows returned value is really a double (with fractions).
uptime = (double) (object_type->PerfTime.QuadPart - value) / (double) object_type->PerfFreq.QuadPart;

On others it's an integer (upcast to double):

struct timespec
	time_t  tv_sec    Seconds. 
	long    tv_nsec   Nanoseconds. 

*uptime = now.tv_sec;