日期增减

 public static List<String> calTimeFormat() {
        int i =0;
        Calendar ca = Calendar.getInstance();//得到一个Calendar的实例
        List<String> timeList = new ArrayList<>();
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        while (i>=-6){
            ca.setTime(new Date()); //设置时间为当前时间
            ca.add(Calendar.DATE, i);
            String format = sdf.format( ca.getTime());
            timeList.add(format);
            i--;
        }
        return timeList;
    }