title: Airflow: Why is there a start_date for operators?
body: <p>I don't understand why do we need a 'start_date' for the operators(task instances). Shouldn't the one that we pass to the DAG suffice?</p>
  
  <p>Also, if the current time is 7th Feb 2018 8.30 am UTC, and now I set the start_date of the dag to 7th Feb 2018 0.00 am with my cron expression for schedule interval being 30 9 * * * (daily at 9.30 am, i.e expecting to run in next 1 hour). Will my DAG run today at 9.30 am or tomorrow (8th Feb at 9.30 am )? </p>