title: Django DateTimeField Defined as blank=True, null=True but doesn't allow null
body: <p>I have a DateTimeField in one of my Django models.</p>
  
  <pre><code>    completed_date = models.DateTimeField('date completed', blank=True, null=True)
  </code></pre>
  
  <p>I've defined it to allow blank and null values. However, when I try to create an instance of the model, I get the following error:</p>
  
  <blockquote>
    <p>IntegrityError at
    /admin/tasks/project/add/</p>
    
    <p>tasks_project.completed_date may not
    be NULL</p>
  </blockquote>
  
  <p>I'm using Django 1.25 and Python 2.7. Anyone know why this is happening? Is there anything I can do to fix this? </p>
  
  <p>I found a <a href="http://code.djangoproject.com/ticket/3172">ticket that describes the same problem</a>, but it was closed as fixed 4 years ago, so I assume it must have been integrated into Django by now!</p>