[求助]求助， 问一个关于const很弱的问题

#include <iostream>
using namespace std;

class mytime
{
    public:
        mytime(int h = 0, int m = 0);
        void gettime(int& h, int& m) const;
        //
        mytime operator+(int min) const;
        
        bool operator==(const mytime& t) const;
    private:
        int hour;
        int minute;
};



mytime::mytime(int h, int m) :
    hour(h), minute(m)
{}
   
void mytime::gettime(int& h, int& m) const{
    h = hour;
    m = minute;
}
///////////////////////////////////////////////////////////////////
//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
mytime mytime::operator+(int min) const
{
    mytime tmp(0,0);
   
    tmp.minute = hour*60 + minute + min;
   
    tmp.hour = tmp.minute / 60;
    tmp.minute %= 60;
   
    return tmp;
}




//== == == == == == == == == == == == == == == == == == == == == == == == == == == == == == == == ==
bool mytime::operator==(const mytime& t) const
{
    if(t.hour == hour && t.minute == minute) return true;
        else return false;
}


int main(){
    int hh, mm;

    mytime a(12, 20);
   
    mytime b;
   
    b=a+100;
   
    b.gettime(hh,mm);
    cout << hh << \":\" << mm;

}

bool mytime::operator==(const mytime& t) const