Timer wont work

I made simple countdown timer in c# and when i put in start time it takes away 5 from this value and then stops (e.g i set start time on 6.0 and it stops at 1.0 and loads level 0). I tried 10 times to rewrite code, but wont help. This is script:

 public float startTime;
 private string currentTime;
 void Update()
 {
     startTime -= Time.deltaTime;
     currentTime = string.Format("{0:0.0}", startTime);
     if (startTime <= 0)
     {
         startTime = 0;
         SceneManager.LoadScene(0);
     }
 }

Even i dont have number 5 nowhere in code lol. Im using unity 5 and vs 15 to edit code