public List<Car> CarInspectionCheck()
{
    List<Car> result = new List<Car>();
    DateTime currentDate = DateTime.Today;

    foreach (var car in allCars)
    {
        if (DateTime.Compare(currentDate, car.TechnicalInspectionDate) => 0 || (car.TechnicalInspectionDate - currentDate).TotalDays <= 30)
        {
            result.add(car);
        }
    }

    return result;
}