Since an administrator of the website I am building is visually impaired, I need to create a custom date field widget rather than use the one provided by Drupal.

I am trying to implement a datelist element.

<?php  namespace Drupalmy_modulePluginFieldFieldWidget;  use DrupalCoreDatetimeDrupalDateTime; use DrupalCoreFieldFieldItemListInterface; use DrupalCoreFieldWidgetBase; use DrupalCoreFormFormStateInterface;  /**  * Accessible date field widget.  *  * @FieldWidget(  *   id = "accessible_date",  *   label = @Translation("Accessible date widget"),  *   field_types = {  *     "datetime"  *   }  * )  */ class AccessibleDate extends WidgetBase {    /**    * {@inheritdoc}    */   public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {     $max_year = date('Y') + 1;     $default_date = new DrupalDateTime();     if (isset($items[$delta]->value)) {       $default_date = new DrupalDateTime($items[$delta]->value);     }      $element += [       '#type' => 'datelist',       '#default_value' => $default_date,       '#date_part_order' => [         'day',         'month',         'year',         'hour',         'minute',       ],       '#date_year_range' => '2000:' . $max_year,     ];      return ['value' => $element];   }  } 

The widget is displayed correctly but when I submit the form to add or edit a node, I have the following error on each of the datelist fields: "The datetime value must be a string."

Do you know how to fix this?