title: Make a change to date fields automatically in Django
body: <p>I have 2 date fields and I want to update the date automatically.
  The code won't work until I update the updated field by myself.
  How can I make it update itself (updated fields) automatically?</p>
  <pre><code>STATUS_CHOICES = (
  ('P', 'Paid'),
  ('U', 'UNPAID'),
  )
  
  status = models.CharField(
      max_length=1, choices=STATUS_CHOICES)
  updated = models.DateTimeField(default=datetime.now() )
  expiry = models.DateTimeField(default=datetime.now() + timedelta(days=30) )
  
  def save(self):
      if(self.expiry &gt;= self.updated):
          self.status = default = &quot;P&quot;
      else:
          self.status = default = &quot;U&quot;
          self.expiry = default=self.updated+timedelta(days=1)
      
      super(Users, self).save()
  </code></pre>