java 如何获得3个月之前的日期

Date dNow = new Date(); //当前时间
Date dBefore = new Date();
Calendar calendar = Calendar.getInstance(); //得到日历
calendar.setTime(dNow);//把当前时间赋给日历
calendar.add(calendar.MONTH, -3); //设置为前3月
dBefore = calendar.getTime(); //得到前3月的时间

SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); //设置时间格式
String defaultStartDate = sdf.format(dBefore); //格式化前3月的时间
String defaultEndDate = sdf.format(dNow); //格式化当前时间

System.out.println("前3个月的时间是：" + defaultStartDate);
System.out.println("生成的时间是：" + defaultEndDate);

Calendar cal = Calendar.getInstance();
cal.set(Calendar.YEAR, 2001);
cal.set(Calendar.MONTH, Calendar.MAY);
cal.set(Calendar.DAY_OF_MONTH, 31);
cal.add(Calendar.MONTH, -3);//当前时间的三个月前

//输出结果
System.out.println("YEAR: "+cal.get(Calendar.YEAR));
int actualMonth = cal.get(Calendar.MONTH)+1;//月份是从0开始的
System.out.println("MONTH: "+actualMonth);
System.out.println("DAY_OF_MONTH: "+cal.get(Calendar.DAY_OF_MONTH));

cal.set(Calendar.YEAR, 2000);
cal.set(Calendar.MONTH, Calendar.MAY);
cal.set(Calendar.DAY_OF_MONTH, 31);
cal.add(Calendar.MONTH, -3);//当前时间的三个月前

//输出结果
System.out.println("YEAR: "+cal.get(Calendar.YEAR));
actualMonth = cal.get(Calendar.MONTH)+1;//月份是从0开始的
System.out.println("MONTH: "+actualMonth);
System.out.println("DAY_OF_MONTH: "+cal.get(Calendar.DAY_OF_MONTH));

Calendar calendar = Calendar.getInstance();
calendar.add(Calendar.MONTH, -3);