code: public function setDefaultOptions(OptionsResolverInterface $resolver)
      {
          parent::setDefaultOptions($resolver);
          
          $resolver->setDefaults(array(
              'widget'         => 'single_text',
              'widget_options'  => array(
                'showButtonPanel' => true,
                'changeMonth'=> true, 
                'changeYear'=>  true
              ),
          ));
          
          $resolver->setAllowedValues(array(
              'input'     => array(
                  'datetime',
                  'string',
                  'timestamp',
                  'array',
              ),
              'widget'    => array(
                  'single_text',
              ),
          ));
          
      }
docs: {@inheritdoc}