how to make a varied countdown timer

I need a countdown timer which is random from 5-10 seconds.
I have done lots of research and even tired mixing codes.

I am not sure what else to try.
thanks

public float timer;

void Start(){
    timer = Random.Range(5f,10f);
}

void Update (){
    timer -= Time.deltaTime;
    if (timer<0){
        Debug.Log("Time's up");
        timer = Random.Range(5f,10f);
    }
}