title: execution_date jinja resolving as a string
body: <p>I have an airflow dag that uses the following jinja template: <code>"{{ execution_date.astimezone('Etc/GMT+6').subtract(days=1).strftime('%Y-%m-%dT00:00:00') }}"</code></p>
  
  <p>This template works in other dags, and it works when the <code>schedule_interval</code> for the dag is set to <code>timedelta(hours=1)</code>. However, when we set the schedule interval to <code>0 8 * * *</code>, it throws the following traceback at runtime:</p>
  
  <pre><code>Traceback (most recent call last):
    File "/usr/lib/python2.7/site-packages/airflow/models/__init__.py", line 1426, in _run_raw_task
      self.render_templates()
    File "/usr/lib/python2.7/site-packages/airflow/models/__init__.py", line 1790, in render_templates
      rendered_content = rt(attr, content, jinja_context)
    File "/usr/lib/python2.7/site-packages/airflow/models/__init__.py", line 2538, in render_template
      return self.render_template_from_field(attr, content, context, jinja_env)
    File "/usr/lib/python2.7/site-packages/airflow/models/__init__.py", line 2520, in render_template_from_field
      for k, v in list(content.items())}
    File "/usr/lib/python2.7/site-packages/airflow/models/__init__.py", line 2520, in &lt;dictcomp&gt;
      for k, v in list(content.items())}
    File "/usr/lib/python2.7/site-packages/airflow/models/__init__.py", line 2538, in render_template
      return self.render_template_from_field(attr, content, context, jinja_env)
    File "/usr/lib/python2.7/site-packages/airflow/models/__init__.py", line 2514, in render_template_from_field
      result = jinja_env.from_string(content).render(**context)
    File "/usr/lib64/python2.7/site-packages/jinja2/environment.py", line 1008, in render
      return self.environment.handle_exception(exc_info, True)
    File "/usr/lib64/python2.7/site-packages/jinja2/environment.py", line 780, in handle_exception
      reraise(exc_type, exc_value, tb)
    File "&lt;template&gt;", line 1, in top-level template code
  TypeError: astimezone() argument 1 must be datetime.tzinfo, not str
  </code></pre>
  
  <p>It appears the execution date being passed in is a string, not a datetime object; but I am only able to hit this error on this specific dag, and no others. I've tried deleting the dag entirely and recreating it with no luck.</p>