title: How to update exisiting date field in django using existing days field?
body: <p>I want to update existing date field in django using existing days field in the same table.</p>
  
  <p>For example</p>
  
  <pre><code>class Sample(models.Model):
      notify_status = models.BooleanField(default=0)
      next_date = models.DateField(blank=True, null=True)
      days = models.IntegerField(blank=True, null=True)
  </code></pre>
  
  <p>Now I want to add <code>days</code> <code>to next_date</code> where <code>next_date</code> is less than todays date.</p>
  
  <p>I have tried this:</p>
  
  <pre><code>import datetime
  from django.db.models import F
  
  Sample.objects.filter(next_date__lt=today).update(next_date=F('next_date')+datetime.timedelta(days=F('days')))
  </code></pre>
  
  <p>But its not working.
  Here is the output</p>
  
  <pre><code>Something went wrong in period_retrack.py: unsupported type for timedelta days component: F
  </code></pre>
  
  <p>Expected Output
  I want next_date to be increased by the number of days in the same table for every row where next_date is less than current_date i.e, today's date</p>