Countdown

Monobeh with coroutine Timer.cs

[RequireComponent(typeof(TimerMonobeh))]
public class ClientTimerMonobeh : MonoBehaviour
{
    [SerializeField] private int _bonus = 1;
    [SerializeField] private float _period = 0.5f;

    private int _score;
    private TimerMonobeh _timer;

    private void Awake()
    {
        _timer = GetComponent<TimerMonobeh>();
    }

    private void Update()
    {
        if (_timer.ElapsedTime >= _period)
        {
            _score += _bonus;
            _timer.ResetElapsedTime();
        }
    }

    private void OnGUI()
    {
        GUI.color = Color.green;
        GUILayout.Label($"Bonus: {_bonus} score in {_period:0.0} sec.");
        GUILayout.Label($"Countdown: {_timer.ElapsedTime:0.0}");
        GUILayout.Label($"Score: {_score}");

        if (GUILayout.Button("SwitchTimer"))
        {
            SwitchTimer();
        }
    }

    private void SwitchTimer()
    {
        if (_timer.IsRunning)
            _timer.Pause();
        else
            _timer.Run();
    }
}

public class TimerMonobeh : MonoBehaviour
{
    public bool IsRunning { get; private set; }
    public float ElapsedTime { get; private set; }

    public void Run()
    {
        IsRunning = true;
        StartCoroutine(Countdown());
    }

    public void Pause()
    {
        IsRunning = false;
    }

    public void ResetElapsedTime()
    {
        ElapsedTime = default;
    }

    private IEnumerator Countdown()
    {
        while (IsRunning)
        {
            ElapsedTime += Time.deltaTime;
            yield return null;
        }
    }
}

Non monobeh Timer.cs

public class ClientTimer : MonoBehaviour
{
    [SerializeField] private int _bonus = 1;
    [SerializeField] private float _period = 0.5f;

    private int _score;
    private Timer _timer;

    private void Awake()
    {
        _timer = new Timer();
    }

    private void Update()
    {
        if (_timer.IsRunning == false)
            return;
        
        _timer.Countdown(Time.deltaTime);

        if (_timer.ElapsedTime >= _period)
        {
            _score += _bonus;
            _timer.Reset();
        }
    }

    private void OnGUI()
    {
        GUILayout.Label($"Bonus: {_bonus} score in {_period:0.0} sec.");
        GUILayout.Label($"Countdown: {_timer.ElapsedTime:0.0}");
        GUILayout.Label($"Score: {_score}");

        if (GUILayout.Button("SwitchTimer"))
        {
            SwitchTimer();
        }
    }

    private void SwitchTimer()
    {
        if (_timer.IsRunning)
            _timer.Pause();
        else
            _timer.Run();
    }
}

public class Timer
{
    public bool IsRunning { get; private set; }
    public float ElapsedTime { get; private set; }

    public void Countdown(float deltaTime)
    {
        if (IsRunning == false)
            return;

        ElapsedTime += deltaTime;
    }
    
    public void Run()
    {
        IsRunning = true;
    }

    public void Pause()
    {
        IsRunning = false;
    }
    
    public void Reset()
    {
        ElapsedTime = default;
    }
}