ModelAdmin - Custom Filter Field

    public function getSearchContext() {

        $context = parent::getSearchContext();
        $fields = $context->getFields();

        if($this->modelClass == 'MyObject') {

            $fields->push(
                DateField::create('q[WeekNr]')
                    ->setTitle("Week of Year")
            );

        }

        return $context;

    }

public function getList() {

        $list = parent::getList();

        // Always limit by model class, in case you're managing multiple
        if($this->modelClass == 'MyObject') {

            if(isset($params['WeekNr']) && !empty($params['WeekNr'])) {
                $startDate = date('Y-m-d', strtotime($params['WeekNr'])) . "00:00:00";
                $finishDate = date('Y-m-d', strtotime($params['WeekNr'], "+1 Week")) . "23:59:59";
                $list = $list->where("\"Created\" BETWEN '".$finishDate."' AND '".$finishDate."'");
            }
        }

        return $list;

    }