code: def get_timedelta(self, now=None):
          
          def datetime_to_time(timestamp):
              atime = time.mktime(timestamp.timetuple())
              atime += timestamp.microsecond / 10.0**6
              return atime
          if self.started is not None:
              now = now or datetime.datetime.now()
              started_time = datetime_to_time(self.started)
              now_time = datetime_to_time(now)
              return now_time - started_time
          return None
docs: Returns number of seconds that passed since ``self.started``, as float.
          None is returned if ``self.started`` was not set yet.