models.py

# Field in the page model

    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."
    )

  
# Then in the clean method

    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, SiteNewsPage)) and instance.live and not instance.publication_date:
        instance.publication_date = timezone.now()
        instance.save(update_fields=["publication_date"])