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function mytheme_preprocess_page(&$variables) {
  // Check if this is a node page and specifically of content type "meeting".
  if (isset($variables['node']) && $variables['node']->getType() === 'meeting') {
    if (!empty($variables['node']->field_when->value)) {
      // Convert the date and set the time zone.
      $date = new \DateTime($variables['node']->field_when->value, new \DateTimeZone('UTC'));
      $user_timezone = \Drupal::currentUser()->getTimeZone() ?: 'Europe/Zurich';
      $date->setTimezone(new \DateTimeZone($user_timezone));
      
      // Add the formatted date to the template variables.
      $variables['formatted_when'] = $date->format('Y-m-d H:i:s');
    }
  }
}