Can't preselect time zone in hook_form_alter

Question

I'm writing a custom installation profile/distribution and I want to pre-populate some fields in the 'Configure Site' form (/core/install.php). I have managed to prefill/preselect other fields like (Site name, Site email address and Default country) but cannot preselect the Default time zone field. This is the code I am using:

function mydistribution_form_install_configure_form_alter(&$form, FormStateInterface $form_state) {
  $form['site_information']['site_name']['#default_value'] = 'Dummy Site Name';
  $form['site_information']['site_mail']['#default_value'] = '[email protected]';
  
  $form['regional_settings']['site_default_country']['#default_value'] = 'DE';
  $form['regional_settings']['date_default_timezone']['#default_value'] = 'Europe/Berlin';
}

As you can see in the screenshot, I manage to change the Default country successfully, but not the Default time zone.

The "strange" thing though is that when I inspect the code, I can see that the correct timezone has been selected but the form displays London.

I'm based in the UK so I guess this is how London gets selected.

Does anyone know how to get this fixed? Thank you in advance!

Answer 1

Drupal adds some JS to detect the timezone (through core/misc/timezone.js). This is interfering with your code, but can be removed easily:

unset($form['regional_settings']['date_default_timezone']['#attributes']);

This removes the timezone-detect class from the <select>, stopping it from being processed by the JS.