You should add it through the get_initial
method, something like:
class AppointmentUpdateView(LoginRequiredMixin, UpdateView):
# ...
def get_initial(self):
initial = super(AppointmentUpdateView, self).get_initial()
conv_date = self.get_object().start_date.astimezone(time_zone)
initial["start_time"] = conv_date.strftime(TIME_FORMAT)
return initial