Ruby builds do not produce Process::UPTIME constant on macOS Sierra
Due to the fact that macOS Sierra does not define CLOCK_UPTIME in /usr/include/time.h the UPTIME constant is never generated during the Ruby build.
macOS defines CLOCK_UPTIME_RAW instead as..
clock that increments monotonically, in the same manner as CLOCK_MONOTONIC_RAW, but that does not increment while the system is asleep. The returned value
is identical to the result of mach_absolute_time() after the appropriate mach_timebase conversion is applied.
It appears this is done to provide a selection for uptime value based on sleeping or non sleeping return value.