Date  Calendar 

https://www.cnblogs.com/lxx2014/p/9394841.html

//得到3个月之前的日期
Date date = new Date();
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
Calendar calendar = Calendar.getInstance();    //获取日历对象
calendar.add(calendar.MONTH, -3);            //设置为前3月
calendar.set(Calendar.HOUR_OF_DAY, 0);
calendar.set(Calendar.MINUTE, 0);
calendar.set(Calendar.SECOND, 0);
date = calendar.getTime();                //返回一个由calendar得到的Date对象
String dateStr = dateFormat.format(date);