See https://bugs.ruby-lang.org/issues/8658

2.1 will add an unofficial API to get OS-level clock values like CLOCK_MONOTONIC and gettimeofday. I have commented on the bug about what these are on JVM and how we should probably proceed.

Note that in the developer meeting transcript linked from the above issue, matz says this is not considered "spec" and other implementations do not need to implement it...but I'm sure we'll have to anyway since there's no alternative.