Do this:

```
-        <a href="{{ request.get_full_path()|urlparams(date_start=feedback.created) }}">
+        {% set feedback_date = convert_date_string(feedback.created) %}
+        <a href="{{ request.get_full_path()|urlparams(date_start=feedback_date) }}">
```

That'll use the helper function I just landed in 32faec2.