title: Temporarily disable auto_now / auto_now_add
body: <p>I have a model like this:</p>
  
  <pre><code>class FooBar(models.Model):
      createtime = models.DateTimeField(auto_now_add=True)
      lastupdatetime = models.DateTimeField(auto_now=True)
  </code></pre>
  
  <p>I want to overwrite the two date fields for some model instances (used when migrating data). The current solution looks like this:</p>
  
  <pre><code>for field in new_entry._meta.local_fields:
      if field.name == "lastupdatetime":
          field.auto_now = False
      elif field.name == "createtime":
          field.auto_now_add = False
  
  new_entry.createtime = date
  new_entry.lastupdatetime = date
  new_entry.save()
  
  for field in new_entry._meta.local_fields:
      if field.name == "lastupdatetime":
          field.auto_now = True
      elif field.name == "createtime":
          field.auto_now_add = True
  </code></pre>
  
  <p>Is there a better solution?</p>