def shopping_filter_calendar(self):
    qs = get_users_purchases(self)
    week = self.request.GET.get('week')

    if _is_valid(week):
        qs = qs.filter(date__range=week)

    return qs

шаблон:
<form method="GET" action=".">
        <button type="submit" class="btn btn-primary" name="week"
                value="[timezone.now() - timezone.timedelta(7), timezone.now()]">Week</button>
    {% for s in statistics %}

       <p>{{ s.cost }}$, {{ s.type }}, {{ s.user }}, {{ s.date }}</p>

    {% endfor %}
    </form>

Ошибка
ValidationError at /
['“[” value has an invalid format. It must be in YYYY-MM-DD HH:MM[:ss[.uuuuuu]][TZ] format.']