title: Problems filtering django datetime field by month and day
body: <p>Can someone explain to me why the following filters are not working at the month and day level?  Filtering by year seems to work, but not the other two.</p>
  
  <pre><code>&gt;&gt;&gt; clicks.count()
  36
  &gt;&gt;&gt; date = clicks[0].created
  &gt;&gt;&gt; date.month
  2
  &gt;&gt;&gt; date.year
  2014
  &gt;&gt;&gt; date.day
  1
  &gt;&gt;&gt; clicks.filter(created__month=2)
  []
  &gt;&gt;&gt; clicks.filter(created__month=02)
  []
  &gt;&gt;&gt; clicks.filter(created__month='02')
  []
  &gt;&gt;&gt; clicks.filter(created__month='2')
  []
  &gt;&gt;&gt; clicks.filter(created__month=date.month)
  []
  &gt;&gt;&gt; clicks.filter(created__day=date.day)
  []
  </code></pre>
  
  <p>A quick update to demonstrate that I am getting the same behavior before creating and dealing with a queryset:</p>
  
  <pre><code>&gt;&gt;&gt; clicks = PreviewClick.objects.filter(created__month = 2)
  &gt;&gt;&gt; clicks.count()
  0
  &gt;&gt;&gt; clicks = PreviewClick.objects.filter(created__month = 02)
  &gt;&gt;&gt; clicks.count()
  0
  &gt;&gt;&gt; clicks = PreviewClick.objects.filter(created__month = '02')
  &gt;&gt;&gt; clicks.count()
  0
  &gt;&gt;&gt; clicks = PreviewClick.objects.filter(created__month = '2')
  &gt;&gt;&gt; clicks.count()
  0
  </code></pre>
  
  <p>Here's more food for thought:</p>
  
  <pre><code>&gt;&gt;&gt; clicks = PreviewClick.objects.all()
  &gt;&gt;&gt; counter = 0
  &gt;&gt;&gt; for click in clicks:
  ...      if click.created.month == 2:
  ...           counter += 1
  ... 
  &gt;&gt;&gt; counter
  35
  </code></pre>