Android/Java で日付の差を日数で計算するにはどうすればよいですか?

// Calculate the difference in milliseconds
long diff = today.getTimeInMillis() - thatDay.getTimeInMillis();

// Convert the difference to days
long days = diff / (24 * 60 * 60 * 1000);

SimpleDateFormat formatter = new SimpleDateFormat("yyyy/MM/dd");
Date d = null;
try {
    d = formatter.parse(strThatDay); // catch exception
} catch (ParseException e) {
    e.printStackTrace();
}

Calendar thatDay = Calendar.getInstance();
thatDay.setTime(d);