class DateRangePlainFormatter

class DateRangePlainFormatter extends DateTimePlainFormatter {
    use DateTimeRangeTrait;
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        return [
            'separator' => '-',
        ] + parent::defaultSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        $separator = $this->getSetting('separator');
        foreach ($items as $delta => $item) {
            if (!empty($item->start_date) && !empty($item->end_date)) {
                
                /** @var \Drupal\Core\Datetime\DrupalDateTime $start_date */
                $start_date = $item->start_date;
                
                /** @var \Drupal\Core\Datetime\DrupalDateTime $end_date */
                $end_date = $item->end_date;
                if ($start_date->getTimestamp() !== $end_date->getTimestamp()) {
                    $elements[$delta] = [
                        'start_date' => $this->buildDate($start_date),
                        'separator' => [
                            '#plain_text' => ' ' . $separator . ' ',
                        ],
                        'end_date' => $this->buildDate($end_date),
                    ];
                }
                else {
                    $elements[$delta] = $this->buildDate($start_date);
                    if (!empty($item->_attributes)) {
                        $elements[$delta]['#attributes'] += $item->_attributes;
                        // Unset field item attributes since they have been included in the
                        // formatter output and should not be rendered in the field template.
                        unset($item->_attributes);
                    }
                }
            }
        }
        return $elements;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $form = parent::settingsForm($form, $form_state);
        $form['separator'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Date separator'),
            '#description' => $this->t('The string to separate the start and end dates'),
            '#default_value' => $this->getSetting('separator'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = parent::settingsSummary();
        if ($separator = $this->getSetting('separator')) {
            $summary[] = $this->t('Separator: %separator', [
                '%separator' => $separator,
            ]);
        }
        return $summary;
    }

}