form_valid updateview - django-models

Question

Good day dear Community!
my Model:
class CheckList(models.Model):
author = models.ForeignKey(User, null=True, on_delete=models.SET_NULL)
...
datecompleted = models.DateTimeField(null=True)
post_date = models.DateTimeField(auto_now=True)
form.py:
class CompletionChecklistForm(forms.ModelForm):
class Meta:
model = CheckList
fields = ('vessels_name', 'datecompleted')
now I'd like to update 'datecompleted' field:
views.py:
class CompleteItemView(UpdateView):
model = CheckList
form_class = CompletionChecklistForm
template_name = 'completion_checklist.html'
success_url = reverse_lazy('home')
def form_valid(self, form):
form.instance.datecompleted = timezone.now()
form.instance.save()
return super(CompleteItemView, self).form_valid(form)
Template:
{% block content %}
<h2 class="d-flex justify-content-center">Completion of current Checklist</h2>
<br>
<br>
<container class="d-flex justify-content-center">
<div class="form-group w-50">
<h4>Complete: {{ object.vessels_name }}?</h4><br>
{{ object.datecompleted }}
<form method="post">
{% csrf_token %}
<div class="clearfix">
<button class="btn btn-danger float-left" type="submit">Yes</button>
No, back
</div>
</form>
</div>
</container>
{% endblock %}
There is no error message but and field value doesn't change as well.
Am I right with my View?
Thank you in advance.