class Exam(models.Model):
    subject = models.ForeignKey(Subject)
    date = models.DateTimeField(auto_now=False, auto_now_add=False)
    add_date = models.DateTimeField(auto_now_add=True, auto_now=False)

    @property
    def how_many_days(self) -> int:
        return (self.date - timezone.now()).days

    @property
    def is_past(self) -> bool:
        return self.how_many_days < 0