Adding over 30 days to 1900-01-01 - c++

Question

How can I add over 30 days in C++ to 1900-01-01 date approx. over 1000 days and then format the time_t after the addition to get a non-broken date.
This is what I have tried so far:
int tmp = 1000;
struct std::tm tm;
std::istringstream ss("1900-01-01");
ss >> std::get_time(&tm, "%Y-%m-%d");
tm.tm_mday = tm.tm_mday + tmp;
return mktime(&tm);

Answer 1

In addition to Joseph Larson's very good suggestion to check out the date/time library to use, I'll show how you could get further using your current idea.
You also have much support in std::chrono nowadays so read about that too.
You try to add the days in the wrong domain, to std::tm. Instead, convert the std::tm to time_t and add the days to that - then convert the result back to std::tm.
Example:
#include <ctime>
#include <iomanip>
#include <iostream>
#include <sstream>
int main() {
int days = 1000;
std::tm tm{};
std::istringstream ss("1900-01-01");
if(ss >> std::get_time(&tm, "%Y-%m-%d")) {
tm.tm_isdst = -1; // let mktime "guess" if DST is effect
// convert to time_t and add 1000 days. 1 day = 24*60*60 seconds
std::time_t result = std::mktime(&tm) + days * 60*60*24;
// back to std::tm
tm = *std::localtime(&result);
// print result
std::cout << std::put_time(&tm, "%Y-%m-%d") << '\n';
}
}
Note: This technique will sometimes get the wrong answer. If for the computer's local time zone the UTC offset at 1900-01-01 is greater than it is at 1900-01-01 + days, then the result will be one day less than it should. This happens (for example) with the IANA time zone America/Anchorage with days == 232. It happens again with Africa/Cairo at days == 273.
A better option is clearly to use the facilities in chrono or Howard Hinnant's date library as demonstrated by Howard.

Answer 2

Date/time handling in C++ is awkward as awkward can be. Howard Hinnant has a great library you may want to look at:
https://github.com/HowardHinnant/date
The problem is complicated. If you use local dates, you can't add a fixed amount of time due to daylight savings time and leap seconds. You could use GMT, but you're still subject to leap seconds.
But Howard's library make make this much easier for you. I'd take a peek.

Answer 3

If you are using the latest Visual Studio 2019, then you have C++20 <chrono> which can solve this problem without the errors associated with the mktime/localtime technique demonstrated in Ted Lyngmo's answser.
#include <chrono>
#include <iostream>
#include <sstream>
int
main()
{
// Hold the amount to add in the type std::chrono::days (a chrono duration)
std::chrono::days days{1000};
// std::chrono::sys_days is a Unix Time chrono::time_point with precision days
std::chrono::sys_days tp;
std::istringstream ss("1900-01-01");
if(ss >> std::chrono::parse("%F", tp)) {
// No need to involve time zones.
// Just add and print out
std::cout << tp + days << '\n'; // 1902-09-28
}
}
This program has the same behavior and output as Ted's answser. But if there's no need to read the "constant" 1900-01-01 out of a stream, then we can do even better. C++20 can make 1900-01-01 a compile-time constant:
#include <chrono>
#include <iostream>
int
main()
{
using namespace std::chrono_literals;
std::chrono::days days{1000};
constexpr std::chrono::sys_days tp = 1900y/01/01; // Compile-time date literal
// Just add and print out
std::cout << tp + days << '\n'; // 1902-09-28
}
These solutions don't involve time zones at all. It is simply adding a number of days to a date. The simplicity makes for efficient code and reduces the chance for errors associated with increased complexity.
If you don't have the latest Visual Studio 2019, or otherwise don't have access to C++20, you can use Howard's free, open-source, header-only "date.h" C++20 chrono preview library referred to in Joseph's answer with nearly identical syntax.
#include "date/date.h"
#include <chrono>
#include <iostream>
int
main()
{
using namespace date::literals;
date::days days{1000};
constexpr date::sys_days tp = 1900_y/01/01; // Compile-time date literal
// Just add and print out
using date::operator<<;
std::cout << tp + days << '\n'; // 1902-09-28
}
The C++20 chrono additions are in namespace date instead of namespace std::chrono.
The year literal is spelled _y instead of y.
The time_point streaming operators won't be found by ADL and have to be manually exposed in namespace date.