title: how to align execution date in a non-catchup setting in airflow?
body: <p>In a catchup setting, I observed that the execution date send to executor is correctly aligned, but when I turn off catchup setting like this:</p>
  
  <pre><code>import airflow
  from airflow import DAG
  from airflow.operators.bash_operator import BashOperator
  from airflow.operators.dummy_operator import DummyOperator
  from datetime import datetime, timedelta
  
  default_args = {
      'owner': 'airflow',
      'depends_on_past': True,
      'start_date': airflow.utils.dates.days_ago(5),
      'schedule_interval': '@daily'
  }
  
  dag = DAG('xiang-01', catchup=False, default_args=default_args)
  
  task = BashOperator(
      task_id='task',
      bash_command='echo "{{ task_instance_key_str }} {{ ts }}" &amp;&amp; sleep 10',
      dag=dag)
  </code></pre>
  
  <p>The execution date is not aligned, for example, the rendered task is:</p>
  
  <pre><code>echo "xiang-01__task__20180909 2018-09-09T22:33:17.961926+00:00" &amp;&amp; sleep 10
  </code></pre>
  
  <p>According to the document, it should be aligned: <a href="https://airflow.apache.org/scheduler.html#backfill-and-catchup" rel="nofollow noreferrer">https://airflow.apache.org/scheduler.html#backfill-and-catchup</a></p>
  
  <p>So what am I missing?</p>
  
  <p><strong>Update:</strong></p>
  
  <p><strong>To be more precise, since my start date is set via <code>days_ago(5)</code>, which is set to the midnight of 5 days ago, start from 00:00:00. What I expected is, the execution date shall also be aligned by midnight, something like <code>2018-09-09T00:00:00</code>, but what I got is a time <code>2018-09-09T22:33:17.961926+00:00</code>, which looks like aligned by the time when I un-paused this DAG.</strong></p>