#include <iostream>
#include <cstdlib>
#include <chrono>   //for class system_clock
#include <ctime>    //for the function localtime
using namespace std;

void travel(int& hour, int& minute, int& second, int distance);

int main()
{
    const auto now {chrono::system_clock::now()};
    const time_t t {chrono::system_clock::to_time_t(now)};
    const tm *const p {localtime(&t)};

    //Get the current hour, minute, and second.
    int h {p->tm_hour};    //0–23
    int m {p->tm_min};     //0–59
    int s {p->tm_sec};     //0–59

    cout << "How many seconds forward do you want to go from "
         << h << ":" << m << ":" << s << "? ";
    int distance {0};
    cin >> distance;

    if (!cin) {
        cerr << "Sorry, that wasn't a number.\n";
        return EXIT_FAILURE;
    }

    if (distance < 0) {
        cerr << "Distance " << distance << " can't be negative.\n";
        return EXIT_FAILURE;
    }

    //The following statement can change the value of h, m, s,
    //but not the value of distance.
    travel(h, m, s, distance);

    cout << "The new time is "
         << h << ":" << m << ":" << s << ".\n";

    return EXIT_SUCCESS;
}

//Move the time distance seconds into the future.
void travel(int& hour, int& minute, int& second, int distance)
{
    /*
    Walk distance seconds into the future.
    Each iteration advances 1 second.
    */

    for (int i {0}; i < distance; ++i) {
        if (second < 59) {
            ++second;
        } else {
            second = 0;
            if (minute < 59) {
                ++minute;
            } else {
                minute = 0;
                if (hour < 23) {
                    ++hour;
                } else {
                    hour = 0;  //Advance into a new day (but not tracking days)
                }
            }
        }
    }
}