code: def _ndays(self, start_date, ndays):
          
          if not getattr(self.args, 'start-date') and not self.config.get('start-date', None):
              raise Exception('start-date must be provided when ndays is used.')
  
          d = date(*map(int, start_date.split('-')))
          d += timedelta(days=ndays)
  
          return d.strftime('%Y-%m-%d')
docs: Compute an end date given a start date and a number of days.