title: peculiar thing about leaving the DateTimeField empty in Django?
body: <p>In the model, I set a <code>DateTimeField</code>'s <code>null</code> and <code>blank</code> to be <code>True</code>. </p>
  
  <p>When I try to save the model, it will complain about a time format error when setting that field to an empty string but it will be successful when I give it <code>None</code>.</p>
  
  <pre><code>a.date = ''
  a.save() #datetime format error
  a.date = None
  a.save() #no error this time
  </code></pre>
  
  <p>Why can't Django auto detect that an empty string is <code>None</code> when I assign it to a field which already allows <code>null</code> and <code>blank</code>?</p>