title: "django.core.exceptions.ValidationError" error
body: <p>I am writing a simple game in Django, all of things were right, but suddenly..., I was encountered by following error:</p>
  
  <ul>
  <li>Django.v = 1.7</li>
  <li>Python.v = 3.4</li>
  </ul>
  
  <p>I don't know what is wrong with these codes:</p>
  
  <pre><code>    (test)alireza@alireza:~/test/test1$ python manage.py syncdb
      Operations to perform:
        Synchronize unmigrated apps: django_admin_bootstrapped, django_admin_bootstrapped_bootstrap3, crispy_forms
        Apply all migrations: contenttypes, admin, auth, arosis, sessions
      Synchronizing apps without migrations:
        Creating tables...
        Installing custom SQL...
        Installing indexes...
      Running migrations:
        Applying arosis.0008_auto_20150212_0826...Traceback (most recent call last):
        File "manage.py", line 10, in &lt;module&gt;
          execute_from_command_line(sys.argv)
        File "/home/alireza/test/lib/python3.4/site-packages/django/core/management/__init__.py", line 385, in execute_from_command_line
          utility.execute()
        File "/home/alireza/test/lib/python3.4/site-
  ...
  ...
  ...
  
          return self.to_python(value)
        File "/home/alireza/test/lib/python3.4/site-packages/django/db/models/fields/__init__.py", line 1252, in to_python
          params={'value': value},
      django.core.exceptions.ValidationError: ["'' value has an invalid format. It must be in YYYY-MM-DD HH:MM[:ss[.uuuuuu]][TZ] format."]
  </code></pre>
  
  <h1>In my models.py:</h1>
  
  <pre><code>class Move(models.Model):
      """docstring for Move"""
      x = models.IntegerField()
      y = models.IntegerField()
      comment = models.CharField(max_length=30)
      game = models.ForeignKey(Game)
      by_first_player = models.BooleanField(default=True)
      timestamp = models.DateTimeField(auto_now_add=True)
  
      def __str__(self):
          return "{}".format(self.comment)
  
      class Meta:
          get_latest_by = 'timestamp'
  
      def player(self):
          return self.game.first_player if self.by_first_player else self.game.second_player
  </code></pre>
  
  <p>I gave 
  <code>auto_now_add=True</code>, </p>
  
  <p>but at first, when I run :</p>
  
  <pre><code>python manage.py makemigrations
  </code></pre>
  
  <p>It asked me for entering a default value for <code>DateTimeField()</code></p>
  
  <p><strong><em>What should I do?</em></strong></p>
  
  <h1>0008_auto_20150212_0826.py</h1>
  
  <pre><code># -*- coding: utf-8 -*-
  from __future__ import unicode_literals
  
  from django.db import models, migrations
  
  
  class Migration(migrations.Migration):
  
      dependencies = [
          ('arosis', '0007_auto_20150211_1844'),
      ]
  
      operations = [
          migrations.AlterModelOptions(
              name='move',
              options={'get_latest_by': 'timestamp'},
          ),
          migrations.AddField(
              model_name='move',
              name='by_first_player',
              field=models.BooleanField(default=True),
              preserve_default=True,
          ),
          migrations.AddField(
              model_name='move',
              name='timestamp',
              field=models.DateTimeField(default='', auto_now_add=True),
              preserve_default=True,
          ),
      ]
  </code></pre>