FormHelper.php

<?php

namespace Drupal\my_module\Form;

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

/**
 * Utility functions and static callbacks for MY MODULE module.
 */
class FormHelper {

  /**
   * Value callback to set default time when time is optional.
   *
   * @param array $element
   *   The form element.
   * @param array $input
   *   An input array (contains `date` and `time` components.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The form state object.
   *
   * @return array
   *   The value array.
   *
   * @see \Drupal\Core\Datetime\Element\Datetime::valueCallback
   */
  public static function setDefaultTime(&$element, $input, FormStateInterface $form_state) {
    if ($input['date'] && empty($input['time'])) {
      $input['time'] = '00:00:00';
    }
    return Datetime::valueCallback($element, $input, $form_state);
  }

}

my_module.module

<?php

/**
 * Implements hook_field_widget_WIDGET_TYPE_form_alter().
 */
function my_module_field_widget_datetime_default_form_alter(&$element, FormStateInterface $form_state, $context) {
  /** @var \Drupal\Core\Field\WidgetInterface $widget */
  $widget = $context['widget'];
  if ($widget->getThirdPartySetting('my_module', 'time_optional', FALSE)) {
    // Time part of the date field is optional, and will default to 00:00.
    // Set a callback function to alter the actual form widget.
    $element['value']['#date_time_callbacks'][] = 'my_module_field_widget_time_optional';
    $element['value']['#value_callback'] = [FormHelper::class, 'setDefaultTime'];
  }
}

/**
 * Implements hook_field_widget_third_party_settings_form().
 */
function my_module_field_widget_third_party_settings_form(WidgetInterface $plugin, FieldDefinitionInterface $field_definition, $form_mode, $form, FormStateInterface $form_state) {
  $element = [];
  if ($plugin->getPluginId() == 'datetime_default') {
    $element['time_optional'] = [
      '#type' => 'checkbox',
      '#title' => t('Time is optional'),
      '#description' => t('If selected, entering the time part of the date is optional and will default to 00:00:00.'),
      '#default_value' => $plugin->getThirdPartySetting('my_module', 'time_optional', FALSE),
    ];
  }
  return $element;
}

/**
 * Implements hook_field_widget_settings_summary_alter().
 */
function my_module_field_widget_settings_summary_alter(&$summary, $context) {
  if ($context['widget']->getPluginId() == 'datetime_default') {
    if ($context['widget']->getThirdPartySetting('my_module', 'time_optional', FALSE)) {
      $summary[] = t('Time is optional');
    }
  }
}

/**
 * Makes the time portion of a datetime form element optional.
 *
 * Note, this isn't in FormHelper since the Datetime element uses a call to
 * `function_exists` rather than `is_callable`.
 *
 * @param array $element
 *   The form element.
 * @param \Drupal\Core\Form\FormStateInterface $form_state
 *   The form state object.
 * @param mixed $date
 *   The date object or array.
 */
function my_module_field_widget_time_optional(&$element, FormStateInterface $form_state, $date) {
  $element['time']['#required'] = FALSE;
}