### Body

Currently `start_date` is mandatory parameter.
For DAGs with `schedule=None` we can relax this requirement as no scheduling calculation needed so the `start_date` parameter isn't used.

### Committer

- [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.
"""
        Make sure DST transitions are properly observed
        """
        local_tz = pendulum.timezone("Europe/Zurich")
        start = local_tz.convert(datetime.datetime(2018, 10, 28, 2, 55), dst_rule=pendulum.PRE_TRANSITION)
        assert start.isoformat() == "2018-10-28T02:55:00+02:00", "Pre-condition: start date is in DST"
        utc = timezone.convert_to_utc(start)
        assert utc.isoformat() == "2018-10-28T00:55:00+00:00", "Pre-condition: correct DST->UTC conversion"
        dag = DAG("tz_dag", start_date=start, schedule="*/5 * * * *")
        _next = dag.following_schedule(utc)
        next_local = local_tz.convert(_next)
        assert _next.isoformat() == "2018-10-28T01:00:00+00:00"
        assert next_local.isoformat() == "2018-10-28T02:00:00+01:00"
        prev = dag.previous_schedule(utc)
        prev_local = local_tz.convert(prev)
        assert prev_local.isoformat() == "2018-10-28T02:50:00+02:00"
        prev = dag.previous_schedule(_next)
        prev_local = local_tz.convert(prev)
        assert prev_local.isoformat() == "2018-10-28T02:55:00+02:00"
        assert prev == utc
    def test_following_previous_schedule_daily_dag_cest_to_cet(self):