              'as this may mean losing timezone information. Use a custom '
              'read-only field and deal with timezone issues explicitly.'
          )
  
          if self.format.lower() == ISO_8601:
              return value.isoformat()
          return value.strftime(self.format)
  
  
  class DurationField(Field):
 
</s> remove         if self.format.lower() == ISO_8601:
 </s> add         if output_format.lower() == ISO_8601: </s> remove         return value.strftime(self.format)
 </s> add         return value.strftime(output_format) </s> remove         if self.format.lower() == ISO_8601:
 </s> add         if output_format.lower() == ISO_8601: </s> remove         return value.strftime(self.format)
 </s> add         return value.strftime(output_format) </s> remove         return value.strftime(self.format)
 </s> add         return value.strftime(output_format)
        if output_format.lower() == ISO_8601:
              'as this may mean losing timezone information. Use a custom '
              'read-only field and deal with timezone issues explicitly.'
          )
  
        if output_format.lower() == ISO_8601:
              return value.isoformat()
          return value.strftime(self.format)
  
  
  class DurationField(Field):
keep keep keep keep replace keep keep keep keep keep
 Access settings lazily, not at module import 
rest_framework/fields.py