code: public function getInputSpecification()
      {
          return [
              'name'       => $this->getName(),
              'required'   => false,
              'filters'    => [
                  [
                      'name'    => Callback::class,
                      'options' => [
                          'callback' => function($date) {
                              // Convert the date to a specific format
                              if (is_array($date)) {
                                  $date = $date['start'] . ' - ' . $date['end'];
                              }
  
                              return $date;
                          }
                      ]
                  ]
              ],
              'validators' => []
          ];
      }
docs: Should return an array specification compatible with {@link Zend\InputFilter\Factory::createInput()}.
  
  @return array