title: Airflow DAG doesn't start based on `start_date`, it starts from now
body: <p>I have an Airflow DAG which I need to backfill, when I change the <code>start_date</code> and run the dag, it doesn't understand the <code>start_date</code> and just starts from the current date.
  I copied my code to a new python file, for example from 'dag_xx.py' to 'dag_xx_backfill.py', and changed the name of dag itself and all the tasks it has. Also, I used the <code>Delete</code> button in the UI to clear the whole state of the dag and start it all over again. But yet, it doesn't start from my desired <code>start_date</code>
  There are some configs in the dag's <code>default_args</code>, like:</p>
  <pre><code>default_args = {
      &quot;owner&quot;: &quot;airflow&quot;,
      &quot;depends_on_past&quot;: False,
      &quot;retries&quot;: 1,
      &quot;retry_delay&quot;: timedelta(minutes=1),
      &quot;catchup&quot;: True
  }
  
  test_dag_backfill = DAG(
      dag_id=&quot;test_dag_backfill&quot;,
      description=&quot;backfill the data&quot;,
      default_args=default_args,
      start_date=datetime(2020, 11, 1, 3, 0, tzinfo=local_tz),
      schedule_interval=&quot;0 * * * *&quot;, # or @hourly
      max_active_runs=1,
  )
  </code></pre>
  <p>As you can see, the <code>start_date</code> is November 1st, but it starts from the current date (December 2nd).
  Do you have any idea what I am missing here?</p>