TimeZoneItem.php

<?php

namespace Drupal\user;

use Drupal\Core\Field\FieldDefinitionInterface;
use Drupal\Core\Field\Plugin\Field\FieldType\StringItem;
use Drupal\user\Entity\User;

/**
 * Defines a custom field item class for the 'timezone' user entity field.
 */
class TimeZoneItem extends StringItem {
    
    /**
     * {@inheritdoc}
     */
    public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
        $timezones = User::getAllowedTimezones();
        // We need to vary the selected timezones since we're generating a sample.
        $key = rand(0, count($timezones) - 1);
        return $timezones[$key];
    }

}