Android/Java를 사용하여 두 날짜의 차이를 일 단위로 계산하는 방법은 무엇입니까?

TextView txtViewData;
txtViewDate.setText("Today is " +
        android.text.format.DateFormat.getDateFormat(this).format(new Date()));

Calendar thatDay = Calendar.getInstance();
thatDay.set(Calendar.DAY_OF_MONTH, 25);
thatDay.set(Calendar.MONTH, 7); // 0-11 so 1 less
thatDay.set(Calendar.YEAR, 1985);

Calendar today = Calendar.getInstance();

long diff = today.getTimeInMillis() - thatDay.getTimeInMillis(); //result in millis

long days = diff / (24 * 60 * 60 * 1000);

String strThatDay = "1985/08/25";
SimpleDateFormat formatter = new SimpleDateFormat("yyyy/MM/dd");
Date d = null;
try {
   d = formatter.parse(strThatDay);//catch exception
} catch (ParseException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
} 


Calendar thatDay = Calendar.getInstance();
thatDay.setTime(d); //rest is the same....