把头两行改为#include<iostream.h>

程序运行正确

#include<iostream>
    class Date {
    public:
        int year,month,day;
    public:
        int yearo() {return year;}
        int montho() {return month;}
        int dayo() {return day;}
        void set(int,int,int);
        void add();
        bool isrun();
        friend std::ostream& operator<<(std::ostream&,const Date&);
    };
    using namespace std;
    void Date::set(int y,int m,int d) {
        year=y;month=m;day=d;}
      ostream& operator<< (ostream& out,const Date& da) {
        return out<<da.year<<"-"<<da.month<<"-"<<"-"<<da.day<<endl;}
    bool Date::isrun() {
        if(year%4==0 && year%100!=0 || year%400==0) return true;
        return false;}

    int main() {
        Date daa;
        int y,m,d;
        cout<<"please input the date "<<endl;
        cin>>y>>m>>d;
        daa.set(y,m,d);
        cout<<daa;
        return 0;
    }

using namespace std;为什么要放后面？