difftime is returning a wrong number

Question

#include <iostream>
#include <ctime>
using namespace std;

int main()
{
    double seconds;    
    struct tm birth = {0}; //10-28-1955
    birth.tm_year = 55;
    birth.tm_mon = 9;
    birth.tm_mday = 28;
    birth.tm_sec = 0;

    struct tm present = {0}; //2-10-2021 
    present.tm_year = 121;
    present.tm_mon = 1;
    present.tm_mday = 10;
    present.tm_sec = 0;

    time_t p1 = mktime(&present);
    time_t b1 = mktime(&birth);
    seconds = (difftime(p1, b1));
    seconds /= 86400;
    cout << "Bill "
         << "Gates- " << seconds << "days" << endl;
}

Output :

Bill Gates- 18668.2days

I'm trying to use <ctime> to try to find the number of days Bill Gates has been alive up to 2-10-2021. I get an answer of 18668.2; that is way off, as it should actually be around 20000 days.

While debugging, everything runs fine until line 21.

When it reaches line 22, b1 becomes -1.

I'm not sure how to fix this. The date I'm putting in for struct birth seems fine.

[`mktime`](https://en.cppreference.com/w/c/chrono/mktime) behavior for dates before the epoch (1970) is implementation defined, see [mktime returns -1 when given a valid struct tm](https://stackoverflow.com/questions/14127013/mktime-returns-1-when-given-a-valid-struct-tm) for example. — dxiv, Feb 27 '21 at 04:39

score 0 · Accepted Answer · answered Feb 27 '21 at 22:32

When it reaches line 22, b1 becomes -1.

Sorry, but your C standard library is not enough to handle your dates. Use (or write) a different library that will allow to represent time before year 1970.

While at it, you are using C++ - no need to use C functions. Try using chrono library that is part of C++ standard library.

difftime is returning a wrong number

1 Answers1