datetimepicker.module

<?php
use Drupal\Component\Serialization\Json;

/**
 * @file
 * DateTimePicker module.
 */

function datetimepicker_element_date_callback(&$element, $form, $form_state) {
  $element['date']['#attributes']['data-datetimepicker-widget'] = TRUE;
  $element['date']['#attributes']['data-datetimepicker-settings'] = Json::encode($element['#datetimepicker_settings']);
}


function datetimepicker_element_time_callback(&$element, $form, $form_state) {
  $settings = array(
    'format' => $element['#date_date_format'],
    'datepicker' => TRUE,
    'timepicker' => FALSE,
    'closeOnDateSelect' => TRUE,
    ) + $element['#datetimepicker_settings'];
  $element['date']['#attributes']['data-datetimepicker-widget'] = TRUE;
  $element['date']['#attributes']['data-datetimepicker-settings'] = Json::encode($settings);

  $settings = array(
    'format' => $element['#date_time_format'],
    'datepicker' => FALSE,
    'timepicker' => TRUE,
    ) + $element['#datetimepicker_settings'];
  $element['time']['#attributes']['data-datetimepicker-widget'] = TRUE;
  $element['time']['#attributes']['data-datetimepicker-settings'] = Json::encode($settings);
}