String day = "2012-10-19";

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
  Date date = new Date();
  try {
   date = sdf.parse(day);
  } catch (ParseException e) {
   e.printStackTrace();
  }
  Calendar calendar = Calendar.getInstance(); //得到日曆
  calendar.setTime(date);//把當前時間賦給日曆
  calendar.add(Calendar.DAY_OF_MONTH, +1);  //設定為後一天
  String day2 = sdf.format(calendar.getTime());