Accumulating timer:

private float time;

void Update()
{
    time += Time.deltaTime;

    if (time >= 5f)
    {
        Debug.Log("5 seconds passed");
        time = 0f;
    }
}

Countdown:

timeRemaining -= Time.deltaTime;
timeRemaining =
    Mathf.Max(
        timeRemaining,
        0f
    );