How can I to override a specific variable from a contributed module?

Question

My goal:

• If a person select a start date the end date is 1 hour later, without the need of selecting the end date (2)

Normally the Form provides 2 selections of dates, start date (1) and end date (2).

I am able to succesfully hide end date (2), in bee_add_reservation_form, with hook_form_alter().

With hook_form_alter() in mymodule.module I modified a form render array.

$end_date is defined in class AddReservationForm of the bee module.

How to alter or override the $end_date in class AddreservationForm? I would like to modify it like so:

    $end_date = clone($start_date);
    $end_date->modify('+1 hour');

Although While doing this within the class this Error occurs:

Error: Call to protected method Drupal\bee\Form\AddReservationForm::getAvailableUnits() from global scope

mymodule.module:

<?php

/**
 * Implements hook_form_alter().
 */
function mymodule_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  if ($form_id == 'bee_add_reservation_form') {
 /**
   * Extends the AddReservationForm class.
   *
   */

    $form['end_date'] = [
        // hides end_date
      '#type' => 'hidden',
        ];
    $form['repeat'] = [
        // hides repeat option
        '#type' => 'hidden',
          ];
    // start date from protected method
   // $start_date = new \DateTime($start_date->format('Y-m-d H:i')); 
    $start_date = AddReservationForm::getAvailableUnits($start_date);

    // end date should be always 1 hour later
    $end_date = clone($start_date);
    $end_date->modify('+1 hour');
      

    
  }

Answer 1

I think hook_form_alter is not the correct place for this. hook_form_alter is called everytime the form is built/created - at this moment your user haven't filled in any start date. So this hook is only helpful to hide the field as you have done.

Instead you should add a custom submit handler in your hook_form_alter

$form['#submit'][] = 'your_custom_submit_handler';

and there in your submit handler you can change the values as you need.

function your_custom_submit_handler(array &$form, FormStateInterface $form_state) {
  $input = $form_state->getUserInput();
  $start_date = $input['start_date'];
   ...
}