title: Convert data type of column to timestamp using explicit cast
body: <p>I'm running the following command to fix the error I've encountered with the Datefield() in my django model.</p>
  
  <pre><code>django.db.utils.ProgrammingError: column "end_date" cannot be cast 
  automatically to type timestamp with time zone
  HINT:  Specify a USING expression to perform the conversion.
  </code></pre>
  
  <p>Here is the line I'm running which doesn't seem to fix the problem:</p>
  
  <pre><code>ALTER TABLE merged_subscription ALTER COLUMN end_date TYPE timestamp USING to_timestamp(col, 'DD-MON-YYY');
  </code></pre>
  
  <p>I'm not very familiar with postresql. Is there something wrong with the logic here?</p>
  
  <p>Here is my Django model for reference:</p>
  
  <pre><code>class Subscription(models.Model):
  start_date = models.DateField()
  end_date = models.DateField()
  date = models.DateTimeField(auto_now_add=True, blank=True)
  issue_one = models.ForeignKey(Issue, blank=True, null=True, related_name='issue_one')
  issue_two = models.ForeignKey(Issue, blank=True, null=True, related_name='issue_two')
  issue_three = models.ForeignKey(Issue, blank=True, null=True, related_name='issue_three')
  def __unicode__(self):
      return unicode(self.start_date)
  </code></pre>