Calendar c = Calendar.getInstance();
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
Date date = sdf.parse("2020-12-01");
c.setTime(date);//设置日历时间
c.add(Calendar.MONTH,-6);//在日历的月份上增加-6个月
System.out.println(sdf.format(c.getTime()));//的到你想要得6个月后的日期