计算N天前的时间,时间戳

    /**
     * 返回指定天前的日期 yyyyMMdd
     * 
     * @param days
     *            days天前的日期
     * 
     * @return
     */
    private static String dateBeforeNow(int days) {
        Calendar cal = Calendar.getInstance();
        SimpleDateFormat format = new SimpleDateFormat("yyyyMMdd");
        cal.setTime(new Date());
        cal.add(Calendar.DATE, -days);
        String time = format.format(cal.getTime());
        return time;
    }

    public static void main(String[] args) {
        String dateBeforeNow = dateBeforeNow(30);//前30天前的日期
        System.out.println(dateBeforeNow);
    }

days天前的时间

    /**
     * 获取days天前的时间戳
     *
     * @param days
     *            天数
     * @return Integer
     */
    private static Integer getTimestampBeforeNow(int days) {
        Calendar cal = Calendar.getInstance();
        cal.setTime(new Date());
        cal.add(Calendar.DATE, -days);
        cal.set(Calendar.HOUR_OF_DAY, 0);
        cal.set(Calendar.MINUTE, 0);
        cal.set(Calendar.SECOND, 0);
        cal.set(Calendar.MILLISECOND, 0);
        long time = cal.getTime().getTime() / 1000;
        return (int) time;
    }

days天前的时间戳