计算当前日期n月前的日期

public static String lastMonth(int allMonth) {
        Date date = new Date();
           int year=Integer.parseInt(new SimpleDateFormat("yyyy").format(date));
           int month=Integer.parseInt(new SimpleDateFormat("MM").format(date))-allMonth;
           int day=Integer.parseInt(new SimpleDateFormat("dd").format(date));
           if(month <= 0){
               int yearFlag = (month*(-1))/12 + 1;
               int monthFlag = (month *(-1))%12;
               year -= yearFlag;
               month=monthFlag*(-1) +12;
           }
           else if(day>28){
               if(month==2){
                   if(year%400==0||(year %4==0&&year%100!=0)){
                       day=29;
                   }else day=28;
               }else if((month==4||month==6||month==9||month==11)&&day==31){
                   day=30;
               }
           }
           String y = year+"";String m ="";String d ="";
           if(month<10) m = "0"+month;
           else m=month+"";
           if(day<10) d = "0"+day;
           else d = day+"";
          
           return y+m+d;
    }