By Vlad K.

#include <iostream>
#include <cstdlib>

using namespace std;

class Time {
private:
    int hours = 0, minutes = 0, seconds = 0;
    int hours1 = 0, minutes1 = 0, seconds1 = 0;
    int totalSec = 0;
public:
    void setHours(int data);
    void setMinutes(int data);
    void setSeconds(int data);
    void setHours1(int data);
    void setMinutes1(int data);
    void setSeconds1(int data);

    void plusData(int hours, int minutes, int seconds, int hours1, int minutes1, int seconds1);
    void minusData(int hours, int minutes, int seconds, int hours1, int minutes1, int seconds1);
    void showResult ();

    Time(string data, string data1) {
        char buf[3] = "";
        buf[0] = data[0];
        buf[1] = data[1];
        setHours(atoi(buf));
        buf[0] = data[3];
        buf[1] = data[4];
        setMinutes(atoi(buf));
        buf[0] = data[6];
        buf[1] = data[7];
        setSeconds(atoi(buf));

        buf[0] = data1[0];
        buf[1] = data1[1];
        setHours1(atoi(buf));
        buf[0] = data1[3];
        buf[1] = data1[4];
        setMinutes1(atoi(buf));
        buf[0] = data1[6];
        buf[1] = data1[7];
        setSeconds1(atoi(buf));

        plusData(hours, minutes, seconds, hours1, minutes1, seconds1);
    }
};

void Time::setHours(int data) {
    hours = data;
}

void Time::setMinutes(int data) {
    minutes = data;
}

void Time::setSeconds(int data) {
    seconds = data;
}

void Time::setHours1(int data) {
    hours1 = data;
}

void Time::setMinutes1(int data) {
    minutes1 = data;
}

void Time::setSeconds1(int data) {
    seconds1 = data;
}

void Time::plusData(int hours, int minutes, int seconds, int hours1, int minutes1, int seconds1) {
    int totalSeconds = 0;
    totalSeconds = hours * 3600 + hours1 * 3600 + minutes * 60 + minutes1 * 60 + seconds + seconds1;
    totalSec = totalSeconds;
}

void Time::minusData(int hours, int minutes, int seconds, int hours1, int minutes1, int seconds1) {
    int totalSeconds = 0;
    totalSeconds = hours * 3600 - hours1 * 3600 + minutes * 60 - minutes1 * 60 + seconds - seconds1;
    totalSec = totalSeconds;
}

void Time::showResult () {
    int hoursTotal = 0, minutesTotal = 0, secondsTotal = 0;
    cout << totalSec;
    hoursTotal = totalSec / 3600;
    totalSec = totalSec % 3600;
    minutes = totalSec / 60;
    totalSec = totalSec % 60;
    secondsTotal = totalSec;
    cout << hoursTotal << ":" << minutesTotal << ":" << secondsTotal << endl;

}

int main()
{
    setlocale(LC_ALL, "rus");
    cout << "Введите время в формате чч:мм:сс" << endl;
    string str, str2;
    cin >> str; cin >> str2;
    Time time (str, str2);
    time.showResult();
    return 0;
}