Calculate past date using Months value - c++

Question

I want to calculate past date from today based on Months input.
Like today is 29-08-2019, 6 months before it was be 29-02-2019.
User inputs would be number of months. It could be 6, 8, 18, 30, 60....
I want to calculate exact complete date. I have tried below code which helps me to get Date for current and past year, but I am looking some solution to get date for months value which is much higher.
time_t now = time( NULL);
struct tm now_tm = *localtime( &now);
int inDuration = 0;
std::cout << "Add Duration..." << std::endl;
std::cin >> inDuration; //month value. looking for solution when mnth value is more then month in current and previous year.
int crnMonth = now_tm.tm_mon+1;
int pastDay = now_tm.tm_mday;
int pastMonth = 0;
int pastYear = now_tm.tm_year + 1900;
if(inDuration > crnMonth)
{
pastMonth = (12-(inDuration-crnMonth));
pastYear = (now_tm.tm_year + 1900)-1;
}
else
{
pastMonth = crnMonth-inDuration;
}
printf("%d-%d-%d", pastDay, pastMonth, pastYear);

Answer 1

This is very easy using Howard Hinnant's free, open-source date/time library:
#include "date/tz.h"
#include <iostream>
int
main()
{
using namespace date;
using namespace std::chrono;
// Get current local date
auto now_utc = floor<seconds>(system_clock::now());
auto now_local = zoned_seconds{current_zone(), now_utc}.get_local_time();
year_month_day crnDate{floor<days>(now_local)};
// Get number of months
int inDuration = 0;
std::cout << "Add Duration..." << std::endl;
std::cin >> inDuration;
// Compute past date
auto pastDate = crnDate - months{inDuration};
if (!pastDate.ok())
pastDate = pastDate.year()/pastDate.month()/last;
std::cout << pastDate << '\n';
}
Example output:
35
2016-09-29
The first few lines get the current time (in UTC), and then convert that to your local time, and then converts the local time to a {year, month, day} structure.
Then the number of desired months is obtained from the user as in your question.
Finally, the input is converted to a months duration and that is subtracted from the current date. If the current date is near the end of a month, it is possible that the past date won't be a valid date (e.g. September 31). If this happens, one has to decide on a policy of what to do. The above example chooses to "snap" to the end of the month. Other policies such as overflowing into the next month are also possible.
Some installation is required for the local time zone support.
If getting the current date in UTC is good enough, then no time zone support is required and you could use the header-only library "date/date.h" (requires no installation). Just #include "date/date.h" instead of #include "date/tz.h", and change the first 3 lines to:
// Get current UTC date
year_month_day crnDate{floor<days>(system_clock::now())};
And the rest of the program works with the same code.