減去日曆

要獲得兩個 Calendar 之間的差異，請使用 getTimeInMillis() 方法：

Calendar c1 = Calendar.getInstance();
Calendar c2 = Calendar.getInstance();
c2.set(Calendar.DATE, c2.get(Calendar.DATE) + 1);

System.out.println(c2.getTimeInMillis() - c1.getTimeInMillis()); //outputs 86400000 (24 * 60 * 60 * 1000)