Dylan's comment requires a timer in both the key fob and in the car. The key fob to delay transmission of the challenge response, the car to check if there isn't too much delay in the challenge / response pair.
You really need a timer in the key fob, as the processor in the key fob is often so slow (for battery / cost reasons) that an extra couple clock cycles somewhere would throw off the timing enough to make it fail.
Dylan's comment requires a timer in both the key fob and in the car. The key fob to delay transmission of the challenge response, the car to check if there isn't too much delay in the challenge / response pair.
You really need a timer in the key fob, as the processor in the key fob is often so slow (for battery / cost reasons) that an extra couple clock cycles somewhere would throw off the timing enough to make it fail.