code: protected long elapsed ()
      {
          long current = current();
          if (_driftRatio != 1.0) {
              long elapsed = current - _priorCurrent;
              _startStamp += (elapsed - (elapsed * _driftRatio));
          }
          _priorCurrent = current;
  
          return current - _startStamp;
      }
docs: Returns the difference between _startStamp and current()