Sometime when I subract a value to hit zero I get a -1

When you are subtracting floats using for instance Time.deltaTime, it will never land on 0, it will always overshoot. So in your code you check if its smaller than 0 instead of checking if its equal, because that will not work. Sometimes it might overshoot so much that it will be rounded to -1 instead of 0. There isn't a single way to correct this. The only thing that is sure, is that you have to handle it on the moment it happens. So when your if statement checks if it is smaller than 0 and its true, just set the value to 0:

  if(timerValue < 0)
     {
         timerValue = 0;
         //Code...
     }

If it is a timer you are using, you could also have a float timer and an int timer. The float timer will be the actual counter, and the int timer will be a rounded version of the float timer. That way it will always be exactly 0 and the problem is gone: float timer = 10; int roundedTimer;

 public void Update()
 {
     timer -= 1 * Time.deltaTime;
     roundedTimer = Mathf.FloorToInt(timer);
     
     if(roundedTimer == 0)
     {
         //Do something...
     }
 }

A side effect of this though, is that the rounded value (depending on if you Ceil, Floor or Round) will never be the same as the actual value and will be wrong by 0.5 seconds max. Maybe if you don't have an if statement but you still don't want it to overshoot, you could use something like this:

 float timer = 10;
 
     public void Update()
     {
         timer -= 1 * Time.deltaTime;
         timer = Mathf.Max(timer, 0);
         
         //Do something
     }

So as you can see, there are many ways to handle the situation. There is no way you must do it. Just make sure you handle it, before you use the data.