title: Django 1.4 TimeField migration fails on PostgreSQL
body: <p>I edited two fields on a model and changed them from <code>IntegerField</code>s to <code>TimeField</code>s:</p>
  
  <pre><code>class Model(models.Model):
      start_time = models.TimeField()
      end_time = models.TimeField()
  </code></pre>
  
  <p>I'm using these two fields to save a naive time, which is not related to any geographical notion of time, and thus has no real 'time zone' (think something similar to race times). My local database is PostgreSQL.</p>
  
  <p>However, the south migration generated from this change fails with the following error:</p>
  
  <pre><code>&gt; main:0005_auto__chg_field_model_start_time__chg_field_model_end_time
  FATAL ERROR - The following SQL query failed: ALTER TABLE "main_model" ALTER COLUMN "start_time" TYPE time, ALTER COLUMN "start_time" SET NOT NULL, ALTER COLUMN "start_time" DROP DEFAULT;
  
  ...
  
  File ".../lib/python2.7/site-packages/django/db/backends/postgresql_psycopg2/base.py", line 52, in execute
      return self.cursor.execute(query, args)
  django.db.utils.DatabaseError: column "start_time" cannot be cast to type time without time zone
  </code></pre>
  
  <p>The failed migration has this:</p>
  
  <pre><code>class Migration(SchemaMigration):
  
      def forwards(self, orm):
  
          # Changing field 'Model.start_time'
          db.alter_column('main_model', 'start_time', self.gf('django.db.models.fields.TimeField')())
  
          # Changing field 'Model.end_time'
          db.alter_column('main_model', 'end_time', self.gf('django.db.models.fields.TimeField')())
  </code></pre>
  
  <p>Any idea on how to make postgres happy about this migration?</p>
  
  <p>P.S. I'm in the midst of development, so I really don't care about any data migrations. You may assume the DB is empty.</p>