Java 按指定的天数计算日期

我使用安卓工作室。我想计算从我指定的日期算起280天后的一天

Calendar thatDay = Calendar.getInstance(); //today date
thatDay.set(Calendar.DAY_OF_MONTH,selectedday);
thatDay.set(Calendar.MONTH,selectedmonth-1); // 0-11 so 1 less
thatDay.set(Calendar.YEAR, selectedyear);


Calendar afterday= Calendar.getInstance(); //280 after date
afterday.set(Calendar.DAY_OF_MONTH,selectedday+10);
afterday.set(Calendar.MONTH,selectedmonth+8); 
afterday.set(Calendar.YEAR, selectedyear);

Log.e(" After Day: ", " days:" +  (afterday.getTimeInMillis()- thatDay.getTimeInMillis())/ (24 * 60 * 60 * 1000));

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日志输出变量。有时285283279种

简单地

cal.add（Calendar.DATE，280）
这是另一个lib，非常简单，谢谢：）
    LocalDate myDate = LocalDate.now();
    LocalDate datePlusDays = myDate.plusDays(280);
    System.out.println("Today Plus 280 Days: " + datePlusDays);