String aa="2016-01-05";
		SimpleDateFormat sf=new SimpleDateFormat("yyyy-MM-dd");
		Calendar ca=Calendar.getInstance();
		ca.add(Calendar.DAY_OF_YEAR, -15);
		System.out.println(sf.format(ca.getTime()));
		ca1.roll(Calendar.DAY_OF_YEAR, -15);
		System.out.println(sf.format(ca1.getTime()));

輸入分別為2015-12-20

                    2016-12-20

Roll方法只能在當前年份進行day計算

Add方法能進行跨年計算