How to add zero on the left on a int field using java

i'm trying to compare a personalnumber with the current date to get the persons age.

The current problem is that if my personalnumber has a zero in it, it gets removed so i can't parse it.

        for (int i = 0; i <= personList.size(); i++) {

            SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd");
            String date = formatter.format(new Date());

            String temp = personList.get(i).getCPR();
            int year = Integer.parseInt((temp.substring(4, 6)));
            if (year < 20) {
                year += 2000;
            } else {
                year += 1900;
            }

            int month = Integer.parseInt(temp.substring(2, 4));
            int day = Integer.parseInt(temp.substring(0, 2));

            /*if (month if month don't start with zero){
                    add 0 to month on the left
                }
                    same goes for day*/


            String birthday = year + "-" + month + "-" + day;

            LocalDate date1 = LocalDate.parse(birthday);
            LocalDate date2 = LocalDate.parse(date);

            long age = date1.until(date2, ChronoUnit.YEARS);

            System.out.println(age);
        }

this is the error i'm getting

Exception in thread "main" java.time.format.DateTimeParseException: Text '1995-1-13' could not be parsed at index 5

I want 1995-1-13 to read 1995-01-13