django split datetime fields

gistfile1.py

#models.py

class Activity(models.Model):
    name = models.CharField()
    start = models.DateTimeField()
    end = models.DateTimeField()


#forms.py

class ActivityForm(forms.ModelForm):
    activity_date = forms.DateField()
    start_time = forms.TimeField()
    end_time = forms.TimeField()

    class Meta:
        model = Activity
        fields = ('name', 'activity_date', 'start_time', 'end_time',)

    def __init__(self, *args, **kwargs):
        super(ActivityForm, self).__init__(*args, **kwargs)

        if kwargs['instance']:
            self.fields['activity_date'].initial = kwargs['instance'].start.date()
            self.fields['start_time'].initial = kwargs['instance'].start.time()
            self.fields['end_time'].initial = kwargs['instance'].end.time()

    def save(self, commit=True):
        model = super(ActivityForm, self).save(commit=False)
        model.start = datetime.combine(self.cleaned_data['activity_date'], self.cleaned_data['start_time'])
        model.end = datetime.combine(self.cleaned_data['activity_date'], self.cleaned_data['end_time'])

        if commit:
            model.save()

        return model