unity subtract float every second

                
                        public float tr = 0f;
  public float hd = 3f;
   
    void Start()
    {
        tr -= holdDur;
    }
    void Update()
    {

        if (Input.GetButtonDown("button"))
        {
            tr = Time.time; 
        }
        if (Time.time < tr + hd)
        {
            RAM(pw);
        }
    }