:class:`timedelta` with the run-time, None if the Task is not running

def get_runtime(self):
        """:class:`timedelta` with the run-time, None if the Task is not running"""
        if self.start is None: return None

        if self.end is None:
            delta = datetime.datetime.now() - self.start
        else:
            delta = self.end - self.start

        return MyTimedelta.as_timedelta(delta)