datetime_tweaks.module

<?php

/**
 * @file
 * Main module functions.
 */

use Drupal\Core\Datetime\DrupalDateTime;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Datetime\Element\Datetime;

/**
 * Implements hook_element_info_alter().
 */
function datetime_tweaks_element_info_alter(array &$types) {
  $types['datetime']['#value_callback'] = 'datetime_tweaks_datetime_value';
  $types['datetime']['#process'][] = 'datetime_tweaks_datetime_set_format';
}

/**
 * Element process callback for datetime fields.
 */
function datetime_tweaks_datetime_set_format($element) {
  // Use d/m/Y format.
  $element['date']['#attributes']['data-drupal-date-format'] = ['d/m/Y'];
  $element['date']['#date_date_format'] = 'd/m/Y';
  $element['date']['#attributes']['title'] = t('Enter a valid date - e.g. @format', [
    '@format' => (new \DateTime())->format('d/m/Y'),
  ]);
  $element['time']['#attributes']['title'] = t('Enter a valid time - e.g. @format', [
    '@format' => (new \DateTime())->format('h:i'),
  ]);
  // Remove seconds in browsers that support HTML5 type=date.
  $element['time']['#attributes']['step'] = 60;
  $element['#attached']['library'][] = 'datetime_tweaks/default_date';
  return $element;
}

/**
 * Element validate callback for browsers that don't support HTML5 type=date.
 */
function datetime_tweaks_datetime_value(&$element, $input, FormStateInterface $form_state) {
  if ($input !== FALSE) {
    try {
      if ($date = DrupalDateTime::createFromFormat('d/m/Y', $input['date'], !empty($element['#date_timezone']) ? $element['#date_timezone'] : NULL)) {
        // Core expects incoming values in Y-m-d format for HTML5 date elements.
        $input['date'] = $date->format('Y-m-d');
      }
    }
    catch (\Exception $e) {
      // Date is not in d/m/Y format - nothing to do.
    }
  }
  return Datetime::valueCallback($element, $input, $form_state);
}