How to correctly add the date to the date? - c++

Question

I'm new in C++, and i have a problem with Unixtime adding. For example, I have a date 8.10.2014 14:49
I need to add 26 days, 12 hours, 44 minutes to it. This is my code:
#include <iostream>
#include <ctime>
struct Date {
int Minute;
int Hour;
int Day;
int Month;
int Year;
};
int main(){
time_t rawtime;
struct tm * timeinfo;
Date startDate;
std::cin >> startDate.Year;
std::cin >> startDate.Month;
std::cin >> startDate.Day;
std::cin >> startDate.Hour;
std::cin >> startDate.Minute;
time ( &rawtime );
timeinfo = localtime ( &rawtime );
timeinfo->tm_year = startDate.Year - 1900;
timeinfo->tm_mon = startDate.Month -1;
timeinfo->tm_mday = startDate.Day;
timeinfo->tm_hour = startDate.Hour;
timeinfo->tm_min = startDate.Minute;
rawtime = mktime ( timeinfo ) + 2555027;
std::cout << rawtime << std::endl;
struct tm *tm = localtime(&rawtime);
char date[80];
strftime(date, sizeof(date), "%Y %m %d %H %M", tm);
std::cout << date;
}
The answers is approximately similar to the truth, but don't correct. Why?

Answer 1

If you have C++11 (or C++14) and a relatively up-to-date compiler, you can use this library:
http://howardhinnant.github.io/date/tz.html
like so:
#include <chrono>
#include <iostream>
#include "tz.h"
int
main()
{
using namespace date;
using namespace std::chrono_literals;
auto zone = current_zone();
auto time = make_zoned(current_zone(), local_days{8_d/10/2014} + 14h + 49min);
time = time.get_sys_time() + days{26} + 12h + 44min;
std::cout << time << '\n';
}
This is roughly equivalent to what you have, which assumes that the date you specified is in your computer's local timezone, and that you also want the results in your computer's local timezone. For me this outputs:
2014-11-04 02:33:00 EST
However you also mentioned "Unixtime", which to me means Unix time. This time is measured in UTC, which lacks complications such as daylight saving time. If it is the intent to do this in Unix time, you can do this more easily and cheaply with this library:
http://howardhinnant.github.io/date/date.html
like this:
#include <chrono>
#include <iostream>
#include "date.h"
int
main()
{
using namespace date;
using namespace std::chrono_literals;
auto time = sys_days{8_d/10/2014} + 14h + 49min + days{26} + 12h + 44min;
std::cout << time << '\n';
}
which will portably output:
2014-11-04 03:33
The use of the "chrono literals" (e.g. 14h) implies C++14. If you are in C++11, you can substitute in the more verbose hours{14} (hours is in namespace std::chrono). If you are in C++03, well, then, never mind. :-)