Program terminates without printing out the coded message [closed]

I tried to run this with a input of 02/29/2001, which ideally should print out the message of You entered an invalid date, but instead, it just terminated after finishing running the program. Where did it go wrong?

  Scanner keyboard = new Scanner(System.in);
        String date;
        String mm, dd, yyyy;
        System.out.print("Please enter a date in mm/dd/yyyy format: ");
        date= keyboard.nextLine();
        mm = date.substring(0, 2);
        
        dd = date.substring(3,5);
    
        yyyy = date.substring(6,10);
    
        int year = Integer.parseInt(yyyy);
        int day = Integer.parseInt(dd);
        int month = Integer.parseInt(mm);
    
        boolean isLeapYear=false;
        if (year % 4 ==0) 
        {
            if (year % 100 !=0)
            isLeapYear=true;
        
        else if (year % 4 ==0 && year % 100 ==0)
            {  if (year % 400 == 0)
                  isLeapYear=true;
            }
        else 
            isLeapYear=false;
        }
        if (isLeapYear=false)
        {
            if(month==2 && day>28)
            System.out.println("You entered an invalid date.");
        }