How can I set up a model/form combination so that the field in the model is initialized to a fixed value and there is no form element displayed when the form is presented to the user?

I just want to initialize Source.updated = datetime.datetime(2000, 1, 1, 0, 0, 0, 0) each time a new Source is created from a form. The user cannot over-ride this initial default. (Subsequent interaction with the app will cause this field value to change, but it's not just auto_now because I want the initial value to be far in the past.)

What I have now is

class Source(Model):
     name = models.CharField(max_length=168, help_text="Source name")
    link = models.URLField(primary_key=True, help_text="Data source link")
    type = models.CharField(max_length=64, help_text="Data source type")
    
     # TODO: add this field to the model.
     # have it initialize to 2000-01-01
    #updated = models.DateTimeField(help_text="Most recent time this source has been updated")

class SourcesForm(forms.ModelForm):

    class Meta:
        model = models.Source
        fields = ['name', 'link', 'type',]
    # TBD: how to configure the form so that
    # it initializes the "updated" field with a fixed value
    # when .save() is called

The logic I'm trying to implement is that: we're getting data from a remote source, so I want to know when the most-recent time this data have been updated is.