Django datetimeinput小部件-验证为日期时间，但在保存之前转换为unix时间

Question

我有一个类似下面的django表单：

class AnnouncementCreationForm(forms.ModelForm):

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.fields['publish_datetime'].widget = forms.DateTimeInput()
        self.fields['publish_datetime'].widget.attrs.update({'placeholder': 'YYYY-MM-DD HH:MM:SS'})
        self.fields['publish_datetime'].required=False
        self.fields['document'].required=False

    class Meta:
        model = CompanyAnnouncement
        exclude = ['company']

模型如下所示：

class CompanyAnnouncement(models.Model):

    class Meta:
        ordering = ['publish_datetime']
        verbose_name = 'Company Announcement'
        verbose_name_plural = 'Company Announcements'

    company = models.ForeignKey(Company, on_delete=models.CASCADE)
    publish_datetime = models.DecimalField(decimal_places=6, max_digits=18, help_text='Leave blank to publish immediately')
    title = models.CharField(max_length=100)
    document = models.FileField(upload_to='company_announcements', help_text='', null=True, blank=True)

我已覆盖publish_datetime的默认输入小部件，以使用DateTimeInput，尽管该值是一个小数。这是因为我希望用户输入日期时间，然后将其转换为Unix时间，然后再将其保存在数据库中。这是在视图中完成的：

class AccouncementCreateView(LoginRequiredMixin, CreateView):

    form_class = AnnouncementCreationForm
    template_name = 'create_announcement.html'
    redirect_unauthenticated_users = True
    success_url = '/users/my_companies'

    def form_valid(self, form):
        form.instance.company = Company.objects.filter(crn=self.kwargs['company_spec_url'])[0]
        print(type(form.instance.publish_datetime)) # Returns 'datetime.datetime'
        if form.instance.publish_datetime:
            form.instance.publish_datetime = form.instance.publish_datetime.timestamp()
        else:
            form.instance.publish_datetime = dt.now().timestamp()
        form.save()
        return super().form_valid(form)

if语句的后半部分（即输入字段为空）可以工作，并且可以成功地将当前的unix时间存储在db中，但前半部分不起作用。该表单抱怨说“请输入一个数字”，表明它从模型定义而不是从输入小部件定义中进行验证。在让转换发生在form.save()之前的视图中之前，如何使datetimeinput小部件验证为正常日期时间？