how to add zero to selected date of birth

I have a button that displays a calendar for the user to pick his/her date of birth but my app crashes if the user pick a date like "9-9-2022" that's if the day or month picked is less than two digits because the backend is expecting something of this kind "09-09-2020". I tried to add zero to the month and day if the selected day or month is less than 10 but it's still not working. Please how should I solve the issue? Here is the code.

 private void setDate(){

        Calendar cal = Calendar.getInstance();
        int year = cal.get(Calendar.YEAR);
        int month = cal.get(Calendar.MONTH);
        int day = cal.get(Calendar.DAY_OF_MONTH);

        android.app.DatePickerDialog dialog = new DatePickerDialog(
                PatientRegistrationActivity.this,
                android.R.style.Theme_Holo_Light_Dialog_MinWidth,
                mDateSetListener,
                year,month,day);
        dialog.getWindow().setBackgroundDrawable(new ColorDrawable(Color.TRANSPARENT));
        dialog.show();

        mDateSetListener = new DatePickerDialog.OnDateSetListener() {
            @Override
            public void onDateSet(DatePicker datePicker, int year, int month, int day) {
                month = month + 1;
                Log.d("Main", "onDateSet: mm/dd/yyy: " + month + "/" + day + "/" + year);
//this is where I want to add 0 to the selected day and time if it's less than 10
                if (month<10){
                    month = Integer.parseInt("0"+month);
                } if (day<10){
                    day = Integer.parseInt("0"+day);
                }

                String date = year + "-" + month + "-" + day;
                btnDateOfBirth.setText(date);
            }
        };
    }