title: datetime.now() is not setting current date
body: <p>I would like to use datetime library to get current date -7 days.</p>
  
  <p>I started with the following incorrect code</p>
  
  <pre><code>from google.appengine.ext import ndb
  from datetime import  datetime, timedelta
  
  class ActivityCount(ndb.Model):
      last_login_date = ndb.DateTimeProperty(default=datetime.now()+timedelta(days=-7))
  </code></pre>
  
  <p>I know that because datetime.now() is being called when your module is initialised and that value is being used for the default.</p>
  
  <p>Hence I am not getting current date 
  and solution for the same is</p>
  
  <pre><code>created = models.DateTimeField(auto_now_add=True)
  </code></pre>
  
  <p>However, I am not able to add +timedelta(days=-7) for the same.</p>
  
  <p>Can you please suggest how can do this ?</p>
  
  <p>Thanks in advance.</p>