<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def save(self, *args, **kwargs):
 """ Set default for ``publish_date``. We can't use ``auto_now_add`` on the field as it will be blank when a blog post is created from the quick blog form in the admin dashboard. """

        if self.publish_date is None:
            self.publish_date = now()
        super(Displayable, self).save(*args, **kwargs)