title: How to perform number of days calculation in Django related field query
body: <p>I have 2 models:</p>
  <pre><code>class Assets(models.Model):
      assetMake = models.CharField(max_length=50,blank=True,)
      assetModel = models.CharField(max_length=50,blank=True,)
  
  class Maintenance(models.Model):
      assetID = models.ForeignKey(Assets, on_delete=models.CASCADE)
      dateRequested = models.DateTimeField(null=True, blank=True)
      dateCompleted = models.DateTimeField(null=True, blank=True)
      complete = models.BooleanField(default = False)
  </code></pre>
  <p>I want to have a view that displays all the Assets in order of when they were last maintained (<code>dateCompleted</code>). Which this accomplishes below:</p>
  <pre><code>from django.db.models import Max
  
  Assets.objects.annotate(
      last_modified=Max('maintenance__dateCompleted')
  ).order_by('-last_modified')
  </code></pre>
  <p>But how can I calculate the number of days from <code>dateCompleted</code> to now so that I can display this value as well.
  Something like:</p>
  <pre><code>Assets.objects.annotate(
       last_modified=Max('maintenance__dateCompleted'),
       days = ((timezone.now() - maintenance__dateCompleted).days
  ).order_by('-last_modified')
  </code></pre>