How to validate date format in maatwebsite excel upload

I have created the import class using model with validation rules. I need to validate the date format for date column as YYYY-MM-DD. The below rules validation is not working. Kindly clarify how to achieve this?

class StudentImport implements ToModel, WithHeadingRow, WithValidation, SkipsOnFailure
{
    use Importable, SkipsFailures;

    public function rules(): array
    {
        return [
                'email' => 'string|email|max:255|exists:users',
        'birth_date' => 'required|dateformat:YYYY-MM-DD', 
        ];
    }
    public function onFailure(Failure ...$failures)
    {
        // Handle the failures how you'd like.
    }
    public function model(array $row)
    {
        //Handle the import code here
    }
}

The below rules validation is not working.

How do I achieve this?