Returns number of seconds that passed since ``self.started``, as float.
        None is returned if ``self.started`` was not set yet.

def get_timedelta(self, now=None):
        """
        Returns number of seconds that passed since ``self.started``, as float.
        None is returned if ``self.started`` was not set yet.
        """
        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