Directly setting values of struct tm's attributes not working

Question

Why does asctime(ptr) return nothing? All the variables of the struct have values. Can someone explain why does this happen?

I also tried using strftime but the result was the same.

#include <iostream>
#include <ctime>
#include <new>
//#include <cstdio>

using namespace std;

int main(int argc,char *argv[])
{
    struct tm *ptr=new struct tm;
    //char buf[50];

    ptr->tm_hour=0;
    ptr->tm_mon=0;
    ptr->tm_year=0;
    ptr->tm_mday=0;
    ptr->tm_sec=0;
    ptr->tm_yday=0;
    ptr->tm_isdst=0;
    ptr->tm_min=0;
    ptr->tm_wday=0;

    cout << asctime(ptr);
    //strftime(buf,sizeof(char)*50,"%D",ptr);
    //printf("%s",buf);

    return 0;
}

Answer 1

The below program works. Remove zero with 1 and it will work.

    struct tm *ptr = new struct tm();
char buf[50];

ptr->tm_hour = 1;
ptr->tm_mon = 1;
ptr->tm_year = 1;
ptr->tm_mday = 1;
ptr->tm_sec = 1;
ptr->tm_yday = 1;
ptr->tm_isdst = 1;
ptr->tm_min = 1;
ptr->tm_wday = 1;
cout << asctime(ptr)

This also works:

 ptr->tm_hour = 0;
ptr->tm_mon = 0;
ptr->tm_year = 0;
ptr->tm_mday = 1;
ptr->tm_sec = 0;
ptr->tm_yday = 0;
ptr->tm_isdst = 0;
ptr->tm_min = 0;
ptr->tm_wday = 0;

cout << asctime(ptr);

Answer 2

The behavior of asctime is undefined if any member of struct tm is outside its normal range.

Especially the behavior is undefined if the calendar day is less than 0 (some implementations handle tm_mday==0 as meaning the last day of the preceding month).

Take a look at http://en.cppreference.com/w/cpp/chrono/c/asctime and http://en.cppreference.com/w/cpp/chrono/c/tm for further details.