Java 中 日期 时间 加减，Date 、Calendar

        DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        //方法1(推荐，功能强大灵活多变)
        Calendar calendar = Calendar.getInstance();
        calendar.add(Calendar.HOUR_OF_DAY, 3);//加3小时，当我减3小时的时候，这里3修改为-3，就可
        System.out.println(dateFormat.format(Calendar.getInstance().getTime()));
        System.out.println(dateFormat.format(calendar.getTime()));

        //方法2
        Date date = new Date();
        date.setTime(date.getTime() + (3 * 60 * 60 * 1000));//加3小时
        System.out.println(dateFormat.format(new Date()));
        System.out.println(dateFormat.format(date));

==========================2019.11.30补充================================

        Date nowDate = new Date();
        try {
            //暂停2秒
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        //可以这样初始化，记住Date的初始化可以带参数
        Date aDate = new Date(nowDate.getTime());

        //可以增加间隔,增加1000毫秒
        Date bDate = new Date(nowDate.getTime() + 1000);

        System.out.println(nowDate.getTime());
        System.out.println(aDate.getTime());
        System.out.println(bDate.getTime());

        DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        //方法1(推荐，功能强大灵活多变)
        Calendar calendar = Calendar.getInstance();
        calendar.add(Calendar.YEAR, -1);//时间减掉一年.
        System.out.println(dateFormat.format(Calendar.getInstance().getTime()));
        System.out.println(dateFormat.format(calendar.getTime()));