trypost/app/Rules/Timezone.php

<?php

declare(strict_types=1);

namespace App\Rules;

use Closure;
use Illuminate\Contracts\Validation\ValidationRule;
use Illuminate\Translation\PotentiallyTranslatedString;

class Timezone implements ValidationRule
{
    /**
     * @param  Closure(string, ?string=): PotentiallyTranslatedString  $fail
     */
    public function validate(string $attribute, mixed $value, Closure $fail): void
    {
        try {
            new \DateTimeZone($value);
        } catch (\Exception) {
            $fail('The :attribute must be a valid timezone.');
        }
    }
}