先放一个错误的例子。。。
虽然结果是对的，但不推荐这么写，也是自己辛苦敲出来的，就放出来记录一下

 String yesterday =DateTime.now().day-1==0||DateTime.now().month-1==0?
                    DateTime.now().day-1==0?
                    DateTime.now().month-1==0?
                    DateTime.now().year.toString()+"."+"12"+"."+"31":
                    DateTime.now().month-1==0?
                    DateTime.now().year.toString()+"."+DateTime.now().month.toString()+"."+(DateTime.now().day-1).toString():
                    DateTime.now().year.toString()+"."+"12"+"."+(DateTime.now().day).toString():
                    DateTime.now().year.toString()+"."+DateTime.now().month.toString()+"."+(DateTime.now().day-1).toString():
                   DateTime.now().year.toString()+"."+(DateTime.now().month-1).toString()+"."+(DateTime.now().day-1).toString();

刚开始不知道DateTime提供了返回的方法，就自己写了一大堆（全部三木运算符）；
在网上偶然看到一篇博客，发现了人本来就提供了方法，为啥还要自己写呢？！！！！

不废话了，直接把整个类展示出来吧，也没啥原理，直接上代码

String calendarBack(int pos){
 String today =DateTime.now().year.toString()+"."+DateTime.now().month.toString()+"."+DateTime.now().day.toString();//2021.8.24   获取今天的日期
 String year=today.substring(0,4);//2021 今年
 String month=today.substring(5,7);//08  这个月
// String day=today.substring(8,10);//24  今天
int nowyear= DateTime.now().year;//今年
int nowmonth= DateTime.now().month;//这个月是几月
int nowday= DateTime.now().day;//今天
  int weekday=DateTime.now().weekday;//今天是本周的第几天
 String  weekstart=new DateTime(nowyear, nowmonth, nowday-weekday+1).toString();//本周一的时间
 String  weekend=new DateTime(nowyear, nowmonth, nowday-weekday+7).toString();//本周日的时间
  String  lastweekstart=new DateTime(nowyear, nowmonth, nowday-weekday-8).toString();//本周一的时间
 String  lastweekend=new DateTime(nowyear, nowmonth, nowday-weekday).toString();//本周日的时间

 if(pos==0){//如果等于0 则返回今天日期
   return DateTime.now().year.toString()+"."+DateTime.now().month.toString()+"."+DateTime.now().day.toString()+"-"+DateTime.now().year.toString()+"."+DateTime.now().month.toString()+"."+DateTime.now().day.toString();
 }else if(pos==1){//如果等于1 则返回昨天日期
   return  (DateTime(nowyear,nowmonth,nowday-1).year).toString()+"."+(DateTime(nowyear,nowmonth,nowday-1).month).toString()+"."+(DateTime(nowyear,nowmonth,nowday-1).day).toString()+"-"+(DateTime(nowyear,nowmonth,nowday-1).year).toString()+"."+(DateTime(nowyear,nowmonth,nowday-1).month).toString()+"."+(DateTime(nowyear,nowmonth-1,nowday-1).day).toString();
 }else if(pos==2){//如果等于2 则返回本周日期
    return weekstart.substring(0,4)+"."+weekstart.substring(5,7)+"."+weekstart.substring(8,10)+"-"+weekend.substring(0,4)+"."+weekend.substring(5,7)+"."+weekend.substring(8,10);
 }else if(pos==3){//如果等于3 则返回上周日期
   return lastweekstart.substring(0,4)+"."+lastweekstart.substring(5,7)+"."+lastweekstart.substring(8,10)+"-"+lastweekend.substring(0,4)+"."+lastweekend.substring(5,7)+"."+lastweekend.substring(8,10);
 }else if(pos==4){//如果等于4则返回本月日期
   return year+"."+month+"1"+"-"+year+"."+month+DateTime(2021,nowmonth+1,0).day.toString();
 }else if(pos==5){//如果等于5 则返回上月日期
   return (DateTime(nowyear,nowmonth-1,nowday).year).toString()+"."+(DateTime(nowyear,nowmonth-1,nowday).month).toString()+"."+"1"+"-"+(DateTime(nowyear,nowmonth-1,nowday).year).toString()+"."+(DateTime(nowyear,nowmonth-1,nowday).month).toString()+"."+(DateTime(nowyear,nowmonth,0).day).toString();
 }
}

可以放在DateUtil里面 直接copy就行