Publication date field

news.py

class NewsPage(Page):
      ...
      publication_date = models.DateTimeField(
        blank=True,
        null=True,
        help_text="This field will be automatically filled in once this news article is published. "
                  "After that, you may edit it. This date is used to sort articles and is displayed on the teaser."
    )

    exclude_fields_in_copy = [
        'go_live_at', 'expire_at', 'expired', 'publication_date'
    ]
      
class NewsPageForm(WagtailAdminPageForm):
    def clean(self):
        cleaned_data = super().clean()

        # Don't let users remove the publication_date from a live article. Unfortunately, the field 'live' defaults
        # to True for a never saved article. But having a live_revision_id appears to be an accurate indicator.
        # This is absolutely required because our code relies on publication_date existing in order to sort News.
        if self.instance.live_revision_id and not cleaned_data.get('publication_date'):
            self.add_error(
                'publication_date',
                ValidationError('You may not remove the publication date from a live news article.')

        return cleaned_data
              
 -----
              
# signals.py              
@receiver(page_published)
def update_publication_date(sender, instance, **kwargs):
    """
    We need an editable 'date this was first published'. We had been using first_published_at but edits
    get lost if someone changes first_published_at and then saves as draft. So make a new field to use for
    display on the teaser and the article itself (and to use for sorting articles). Fill it in here.
    """
    if issubclass(sender, NewsPage) or issubclass(sender, SiteNewsPage):
        if instance.live and not instance.publication_date:
            instance.publication_date = timezone.now()
            instance.save(update_fields=['publication_date'])