code: public final long getIdleStartMilliseconds() {
          long firstIdle = Long.MIN_VALUE;
          for (Executor e : oneOffExecutors) {
              firstIdle = Math.max(firstIdle, e.getIdleStartMilliseconds());
          }
          for (Executor e : executors) {
              firstIdle = Math.max(firstIdle, e.getIdleStartMilliseconds());
          }
          return firstIdle;
      }
docs: Returns the time when this computer last became idle.
  
  <p>
  If this computer is already idle, the return value will point to the
  time in the past since when this computer has been idle.
  
  <p>
  If this computer is busy, the return value will point to the
  time in the future where this computer will be expected to become free.