Elapsed time [µs] between start and stop timestamps. If stop is empty then
        returned time is difference between start and current timestamp.

def elapsed(self):
        """
        Elapsed time [µs] between start and stop timestamps. If stop is empty then
        returned time is difference between start and current timestamp.
        """
        if self._stop is None:
            return timer() - self._start
        return self._stop - self._start