title: How to insert NULL into Django's DateField?
body: <p>I have an optional datefield in my django model:</p>
  
  <pre><code>award_grant_date = models.DateField(null=True, blank=True)
  </code></pre>
  
  <p>When the user leaves the date field blank, NULL is inserted into the DateField, otherwise the date is inserted.</p>
  
  <p>However, if the user does enter a date into the DateField, and the user has selected some other specific data in the form, then I want to insert NULL into the DateField.</p>
  
  <p>So, in my form I have the following code:</p>
  
  <pre><code>    class AwardGrantDetailsForm(forms.Form):
      ...
      award_grant_date = forms.DateField(
      widget=forms.DateInput(attrs={'id':'id_award_grant_date'}),
      input_formats=settings.DATE_INPUT_FORMATS,
      label=_('Date'),
      required=False)
      ...
      elif cd_agdf['award_grant_type'] == 8888 or cd_agdf['award_grant_type'] == 9999:
  
          self.cleaned_data['award_grant_type_description'] = ''
          self.cleaned_data['award_grant_date'] = 'NULL' //this is the issue here! 
  </code></pre>
  
  <p>The error I am getting is:</p>
  
  <pre><code>[u"'NULL' value has an invalid date format. It must be in YYYY-MM-DD format."]
  </code></pre>
  
  <p>I cannot  seem to get the syntax correct to enter the NULL in to the award_grant_date date field.</p>
  
  <p>I am using bootstrap datepicker and django 1.4.</p>
  
  <p>How do I write the code self.cleaned_data['award_grant_date'] = '' to insert the required NULL? I have set the models field to null=True, blank=True!</p>