java获取前一天的日期

				SimpleDateFormat sdf1 = new SimpleDateFormat("yyyy-MM-dd");
                SimpleDateFormat sdf2= new SimpleDateFormat("yyyy-MM-dd 20:00:00");
                Date day = sdf1.parse(String.valueOf(dayNow.split(" ")[0]));
                long ms = day.getTime() - 1*24*3600*1000L;
                Date prevDay = new Date(ms);
                String dayBefore=sdf2.format(prevDay);
                String dayToday=sdf2.format(new Date(day.getTime()));
                System.out.println("昨天20点："+dayBefore);
                System.out.println("今天20点："+dayToday);