Timer problem. When you click on the button in the UI, the timer when it becomes > 15 should turn off the object. What to do?

Question

User interface

public GameObject RELOAD;
public float timer = 0f;
public float cooldown = 15f;

public void Start()
{

}

public void Update()
{
timer += Time.deltaTime;

}
public void OFF()
{
RELOAD.SetActive(true);
timer=0f;

if(timer > cooldown)
{
RELOAD.SetActive(false);
}
}

Answer 1

timer = 0f;
if(timer > cooldown)

What is the check timer?