title: airflow trigger_dag execution_date is the next day, why?
body: <p>Recently I have tested airflow so much that have one problem with <code>execution_date</code> when running <code>airflow trigger_dag &lt;my-dag&gt;</code>.</p>
  
  <p>I have learned that <code>execution_date</code> is not what we think at first time from <a href="https://cwiki.apache.org/confluence/display/AIRFLOW/Common+Pitfalls" rel="noreferrer">here</a>:</p>
  
  <blockquote>
    <p>Airflow was developed as a solution for ETL needs. In the ETL world,
    you typically summarize data. So, if I want to summarize data for
    2016-02-19, I would do it at 2016-02-20 midnight GMT, which would be
    right after all data for 2016-02-19 becomes available.</p>
  </blockquote>
  
  <pre><code>start_date = datetime.combine(datetime.today(),
                                datetime.min.time())
  
  args = {
      "owner": "xigua",
      "start_date": start_date
  }
  dag = DAG(dag_id="hadoopprojects", default_args=args,
            schedule_interval=timedelta(days=1))
  
  
  wait_5m = ops.TimeDeltaSensor(task_id="wait_5m",
                                dag=dag,
                                delta=timedelta(minutes=5))
  </code></pre>
  
  <p>Above codes is the start part of my daily workflow, the first task is a TimeDeltaSensor that waits another 5 minutes before actual work, so this means my dag will be triggered at <code>2016-09-09T00:05:00</code>, <code>2016-09-10T00:05:00</code>... etc.</p>
  
  <p>In Web UI, I can see something like <code>scheduled__2016-09-20T00:00:00</code>, and task is run at <code>2016-09-21T00:00:00</code>, which seems reasonable according to <code>ETL</code> model.</p>
  
  <p>However someday my dag is not triggered for unknown reason, so I trigger it manually, if I trigger it at <code>2016-09-20T00:10:00</code>, then the TimeDeltaSensor will wait until <code>2016-09-21T00:15:00</code> before run. </p>
  
  <p>This is not what I want, I want it to run at <code>2016-09-20T00:15:00</code> not the next day, I have tried passing <code>execution_date</code> through <code>--conf '{"execution_date": "2016-09-20"}'</code>, but it doesn't work.</p>
  
  <p>How should I deal with this issue ?</p>
  
  <pre><code>$ airflow version
  [2016-09-21 17:26:33,654] {__init__.py:36} INFO - Using executor LocalExecutor
    ____________       _____________
   ____    |__( )_________  __/__  /________      __
  ____  /| |_  /__  ___/_  /_ __  /_  __ \_ | /| / /
  ___  ___ |  / _  /   _  __/ _  / / /_/ /_ |/ |/ /
   _/_/  |_/_/  /_/    /_/    /_/  \____/____/|__/
     v1.7.1.3
  </code></pre>