title: Django ORM calculate number of days between two date attributes
body: <h2>Scenario</h2>
  
  <p>I have a table <code>student</code>. it has following attributes</p>
  
  <p><code>name</code>, </p>
  
  <p><code>age</code>, </p>
  
  <p><code>school_passout_date</code>, </p>
  
  <p><code>college_start_date</code></p>
  
  <p>I need a report to know what is the <code>avg number of days</code> student get free between the passing the school and starting college. </p>
  
  <h2>Current approach</h2>
  
  <p>Currently i am irritating over the range of values finding days for each student and getting its avg.</p>
  
  <h2>Problem</h2>
  
  <p>That is highly inefficient when the record set gets bigger. </p>
  
  <h2>Question</h2>
  
  <p>Is there any ability in the Django ORM that gives me totals days between the two dates?</p>
  
  <h2>Possibility</h2>
  
  <p>I am looking for something like this. </p>
  
  <pre><code>Students.objects.filter(school_passed=True, started_college=True).annotate(total_days_between=Count('school_passout_date', 'college_start_date'), Avg_days=Avg('school_passout_date', 'college_start_date'))
  </code></pre>