title: Adding days to datetime
body: <p>I wan't to add to planned date integer. It's always will be from 1 to 10, but having problem with my code.</p>
  
  <pre><code>planned_date = fields.Date(string='Planned Date', required=False,
                             default=fields.Date.today)
  def linedate(self):    
      if line.discount &gt; 5:
         
          daysz = line.product_id.seller_ids[0].delay # it's integer from 1 to 10 always
     
          planned = (line.planned_date + timedelta(days=daysz) ).strftime('%Y-%m-%d')
          line.planned_date =  planned
          print line.planned_date
  </code></pre>
  <p>I'm getting this kind of error</p>
  <blockquote>
  <p>planned = (line.planned_date + timedelta(days=daysz) ).strftime('%Y-%m-%d')</p>
  <p>TypeError: coercing to Unicode: need string or buffer, datetime.timedelta found</p>
  </blockquote>