Django-taggit: how to save after adding tags in save()

def save(self, *args, **kwargs):
    super(Flow, self).save(*args, **kwargs)

    if self.pk:
        self.tags.add(str(date.today().year))
        self.save()

This doesn't work in my model Flow. How can I save the newly added tag into tags of my flow instance successfully? Thanks.

My Flow model:

class Flow(models.Model):
    remarks = models.TextField(null=True, blank=True)
    tags = TaggableManager(blank=True)