Is it possible to make a timer that ignores timescale?

My game sets the timeScale to 0 in between waves so that the player can upgrade weapons and barriers in menus without the next wave starting. However, I also have background music playing that has an intro. I created a second audio clip that does not have an intro which i have set up to start once the timer reaches a certain time. The menu, and subsequently the timeScale freeze, are effecting the timer that tells the audio manager when to play the second clip. Is there a way to make this timer work independently of the timeScale?

public float timer;
    public Sound[] sounds;
    // Start is called before the first frame update
    void Awake()
    {
        foreach (Sound s in sounds)
        {
            s.source = gameObject.AddComponent<AudioSource>();
            s.source.clip = s.clip;

            s.source.volume = s.volume;
            s.source.pitch = s.pitch;
            s.source.loop = s.loop;

            
        }
    }

    void Start()
    {
        Play("Theme");
        timer = 0f;
    }

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

        if (timer >= 240f)
        {
            Play("Theme 2");
        }
    }

    public void Play (string name)
    {
        Sound s = Array.Find(sounds, sound => sound.name == name);
        if (s == null)
        {
            Debug.LogWarning("Sound" + name + "not found!");
            return;
        }
        s.source.Play();
    }

This is the code for reference.

Would Time.unscaledDeltaTime work for you ?
Unlike deltaTime this value is not affected by timeScale.

Another option that many might find useful is WaitForSecondsRealtime. Please check that out