title: Google App Engine computed property date return throws exception
body: <p>I have an issue with Google App Engine (Python) decorator <code>@db.ComputedProperty</code>.</p>
  
  <p>I have a model like :</p>
  
  <pre><code>class Employee(db.Model):
      name = db.StringProperty()
      date_of_join = db.DateProperty()
  
      @db.ComputedProperty
      def date_of_employment(self):
          return self.date_of_join
  </code></pre>
  
  <p>When I try to insert values into the model, it is throwing an exception as:</p>
  
  <pre><code>BadValueError: Unsupported type for property date_of_employment: &lt;type 'datetime.date'&gt;
  </code></pre>
  
  <p>Can't computedProperty return/insert date values?</p>
  
  <h2>Edit</h2>
  
  <p>I have found a solution. Appengine accepts datetime for computed property, not date object:</p>
  
  <pre><code>class Employee(db.Model):
      name = db.StringProperty()
      date_of_join = db.DateProperty()
  
      @db.ComputedProperty
      def date_of_employment(self):
          return datetime.combine(self.date_of_join, datetime.min.time())
  </code></pre>