Question

#include <time.h>
#include <iostream>
int main()
{

        struct tm dayofmonth = {0};
        int iYear = 2012;
        int iMonth = 2; // February

        dayofmonth.tm_year = iYear - 1900;
        dayofmonth.tm_mon =  iMonth;
        dayofmonth.tm_mday = 0;
        dayofmonth.tm_isdst = 0;

        mktime(&dayofmonth);

        std::cout << "Number of days for the month " << dayofmonth.tm_mon << " is " << dayofmonth.tm_mday << std::endl;

}

Needed to write a simple routine, to find the number of days for a given month. However, for mktime, why should I pass the actual month number instead of month number -1.

It is more confusing, that after calling mktime, the tm_mon returns month -1 instead of the original month that was passed.

Answer 1

Because you set tm_mday = 0. The “zeroth” of the month (tm_mon = 2 means March) rolls back to the last day of the previous month (February).

Yes, it is confusing that tm_mday is 1-based whereas tm_mon is 0-based. You get used to it eventually 🙂