class DateTimeDatelistWidget

class DateTimeDatelistWidget extends DateTimeWidgetBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        return [
            'increment' => '15',
            'date_order' => 'YMD',
            'time_type' => '24',
        ] + parent::defaultSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
        $element = parent::formElement($items, $delta, $element, $form, $form_state);
        // Wrap all of the select elements with a fieldset.
        $element['#theme_wrappers'][] = 'fieldset';
        $date_order = $this->getSetting('date_order');
        if ($this->getFieldSetting('datetime_type') == 'datetime') {
            $time_type = $this->getSetting('time_type');
            $increment = $this->getSetting('increment');
        }
        else {
            $time_type = '';
            $increment = '';
        }
        // Set up the date part order array.
        switch ($date_order) {
            case 'YMD':
                $date_part_order = [
                    'year',
                    'month',
                    'day',
                ];
                break;
            case 'MDY':
                $date_part_order = [
                    'month',
                    'day',
                    'year',
                ];
                break;
            case 'DMY':
                $date_part_order = [
                    'day',
                    'month',
                    'year',
                ];
                break;
        }
        switch ($time_type) {
            case '24':
                $date_part_order = array_merge($date_part_order, [
                    'hour',
                    'minute',
                ]);
                break;
            case '12':
                $date_part_order = array_merge($date_part_order, [
                    'hour',
                    'minute',
                    'ampm',
                ]);
                break;
            case 'none':
                break;
        }
        $element['value'] = [
            '#type' => 'datelist',
            '#date_increment' => $increment,
            '#date_part_order' => $date_part_order,
        ] + $element['value'];
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $element = parent::settingsForm($form, $form_state);
        $element['date_order'] = [
            '#type' => 'select',
            '#title' => $this->t('Date part order'),
            '#default_value' => $this->getSetting('date_order'),
            '#options' => [
                'MDY' => $this->t('Month/Day/Year'),
                'DMY' => $this->t('Day/Month/Year'),
                'YMD' => $this->t('Year/Month/Day'),
            ],
        ];
        if ($this->getFieldSetting('datetime_type') == 'datetime') {
            $element['time_type'] = [
                '#type' => 'select',
                '#title' => $this->t('Time type'),
                '#default_value' => $this->getSetting('time_type'),
                '#options' => [
                    '24' => $this->t('24 hour time'),
                    '12' => $this->t('12 hour time'),
                ],
            ];
            $element['increment'] = [
                '#type' => 'select',
                '#title' => $this->t('Time increments'),
                '#default_value' => $this->getSetting('increment'),
                '#options' => [
                    1 => $this->t('1 minute'),
                    5 => $this->t('5 minute'),
                    10 => $this->t('10 minute'),
                    15 => $this->t('15 minute'),
                    30 => $this->t('30 minute'),
                ],
            ];
        }
        else {
            $element['time_type'] = [
                '#type' => 'hidden',
                '#value' => 'none',
            ];
            $element['increment'] = [
                '#type' => 'hidden',
                '#value' => $this->getSetting('increment'),
            ];
        }
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = [];
        $summary[] = $this->t('Date part order: @order', [
            '@order' => $this->getSetting('date_order'),
        ]);
        if ($this->getFieldSetting('datetime_type') == 'datetime') {
            $summary[] = $this->t('Time type: @time_type', [
                '@time_type' => $this->getSetting('time_type'),
            ]);
            $summary[] = $this->t('Time increments: @increment', [
                '@increment' => $this->getSetting('increment'),
            ]);
        }
        return $summary;
    }

}