title: auto_now_add fields override my input
body: <p>Given a model</p>
  
  <pre><code>class Foo(models.Model):
      bar = models.DateTimeField(auto_now_add=True)
  </code></pre>
  
  <p>When I do</p>
  
  <pre><code>my_foo = Foo.objects.create(bar=yesterday)
  </code></pre>
  
  <p>Then <code>my_bar.foo</code> is not yesterday but now. I have to do</p>
  
  <pre><code>my_foo = Foo.objects.create()
  my_foo.bar = yesterday
  my_foo.save()
  </code></pre>
  
  <p>This didn't use to be the case, but is true for Django 1.8.17</p>