My Instant.now() don't work, but when it does i get another problem

Question

This is some basic program that I trying to build to learn some Java before I'm starting in programming courses. I can't get the time to pulloff in the right time interval between Start = Instant.now() and Ends = Instant.now(). But the code works if I use Scanner instead(example: I use a String in the If(String.isEmpty() != Thread.alive()), why does it do that? But then I get a problem with I need to press enter twice, which I makes me so so so frustrated.

class UserThread extends Thread {
       public void run() {
          Random rng = new Random();
          
          int time = rng.nextInt(5000);
          for(int i=1; i <= 1; i++) {
             try {              
                Thread.sleep(time); // sleep/stop a thread for 1 second
                System.out.println("GO!");
      
                }
             catch(InterruptedException e ) {
                    System.out.println("Error ");
             }
          }
       }
    }

public class reaktion{
    
    public static void main(String[] args) throws IOException {
        
        UserThread ut = new UserThread();
        ut.start();
        System.out.println("Ready, set....");
        
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        
        if (reader.ready() && ut.isAlive()) {
                reader.read();
                System.out.println("Too early");
                
        }
        if (reader.ready() != ut.isAlive()) {
                Instant starts = Instant.now();
                reader.read();
                Instant ends = Instant.now();
                System.out.print("Finish! Reaction time : " + Duration.between(starts, ends).toMillis()+"ms");
                            
        }    
    }
}