c++动态时钟问题 为什么不能正确显示？

#include<iostream>
#include<stdlib.h>
#include<time.h>
#include <windows.h>
#include "dos.h"
using namespace std;
class Clock      //定义一个Clock类 里面有时、分、秒
{
public:
    Clock(int a = 0, int b = 0, int c = 0)//定义Clock类构造函数
    {
        Hour = a;
        Minute = b;
        Second = c;
    }
    friend Clock operator+(Clock&t1, Clock&t2);//声明运算符+重载函数
    void ShowTime();//
private:
    int Hour;  //时
    int Minute;//分
    int Second;//秒
};
void Clock::ShowTime()//Clock类函数 输出时分秒
{
    cout << Hour << ":" << Minute << ":" << Second << endl;

}
Clock operator+(Clock &t1, Clock&t2)//定义Clock类运算符+重载函数
{
    Clock t3;
    t3.Second = t1.Second + t2.Second;
    if (t3.Second >= 60)//如果秒大于或等于60秒则结果是取除于60的余数
    {
        t3.Second = t3.Second - 60;
        t3.Minute = t1.Minute + t2.Minute + 1;
        if (t3.Minute >= 60)//如果分大于60则减去模60
        {
            t3.Minute = t3.Minute - 60;
            t3.Hour = t1.Hour + t2.Hour + 1;
        }
        if (t3.Hour >= 24)//如果时大于60则减去模24
        {
            t3.Hour = t3.Hour - 24;
        }
    }
    else//如果秒没有大于或等于60秒
    {
        t3.Minute = t1.Minute + t2.Minute;
        t3.Hour = t1.Hour + t2.Hour + 1;
        if (t3.Minute >= 60)//如果分大于60则减去模60
        {
            t3.Minute = t3.Minute - 60;
            t3.Hour = t1.Hour + t2.Hour + 1;
        }
        if (t3.Hour >= 24)//如果时大于60则减去模24
        {
            t3.Hour = t3.Hour - 24;
        }
    }
    return t3;
}
void main()
{
    Clock t1(8, 30, 6), t2(0, 0, 1), t3;//定义3个Clock类对象
    t3 = t1 + t2;
    t3.ShowTime();//输出时分秒
    Sleep(1000);
    system("cls");
        while (1)
        {
            t3 = t3 + t1;
            t3.ShowTime();//输出时分秒
            Sleep(1000);
            system("cls");
        }

    system("pause");
}