Add to calendar link is not visible on page build with layout builder.

Comment #7 appears to be exactly correct.

The "Add to calendar" field type works in Layout Builder.

However, using the "Add to calendar" checkbox when configuring the display of a Date or Daterange field does not work in Layout Builder.

The bug is that in _addtocalendar_preprocess_field(), the code tries to get settings from the EntityViewDisplay, ignoring whether Layout Builder is in use:

  $entity = $variables['element']['#object'];
  $view_mode = $variables['element']['#view_mode'];
  $field_name = $variables['element']['#field_name'];

  // Determine if the field is multivalued.
  $multivalued = count($variables['items']) > 1;

  // Get the field formatter settings...
  $entity_display = EntityViewDisplay::collectRenderDisplay($entity, $view_mode);
  $field_display = $entity_display->getComponent($field_name);

So that actually points to a workaround. The $view_mode in the code above evaluates to _custom when the field is rendering in Layout Builder. That's not a view mode you can configure through field UI. That means the code will end up getting the field formatter configuration for the default view mode.

If you configure the default view mode for a Date field with the "Add to calendar" checkbox (though Field UI), then the "Add to calendar" button will show up when that Date field is displayed in Layout Builder.

I found a very similar issue for another module that uses third_party_settings to enhance field formatters. #3099580: Doesn't work in layout builder →

The fix there would almost work for us. Unfortunately, we get a first-party setting from the field formatter as well:

$timeZone = (!empty($field_display['settings']['timezone_override'])) ? $field_display['settings']['timezone_override'] : date_default_timezone_get();

I don't see how to get that from $variables directly.