title: How to make Django DateTimefield non required?
body: <p>I know, there is a similiar question, which (<a href="https://stackoverflow.com/questions/11351619/how-to-make-djangos-datetimefield-optional">I have read</a>), but I have a question to that issue.</p>
  
  <p>How my forms.py goes:</p>
  
  <pre><code>class DataForm(forms.ModelForm):
      start_date = forms.CharField(
          widget=widgets.DateTimePicker(attrs={'class': 'form-control', 'style': 'width:90%'}),
          required=False
      )
  ....
  class Meta:
      model = Data   
  </code></pre>
  
  <p>My models.py goes that way:</p>
  
  <pre><code>start_date = models.DateTimeField(null=True, blank=True)
  </code></pre>
  
  <p>And when I try to save that way:</p>
  
  <pre><code>    if form.is_valid():
      form.save()
  </code></pre>
  
  <p>I got this error:</p>
  
  <pre><code>[u"'' value has an invalid format. It must be in YYYY-MM-DD HH:MM[:ss[.uuuuuu]][TZ] format."]
  </code></pre>
  
  <p>What is wrong with that? I want to make start_date field optional.    </p>