difference between two date in C

Question

I am trying to get the difference between two date by using below C code.

but code always giving difference 0. Help me to where i am making mistake.

I am using gcc compiler under linux.

#include <stdio.h>  
#include <time.h>       
int main ()
{
  struct tm start_date;
  struct tm end_date;
  time_t start_time, end_time;
  double seconds;

  start_date.tm_hour = 0;  start_date.tm_min = 0;  start_date.tm_sec = 0;
  start_date.tm_mon = 10; start_date.tm_mday = 15; start_date.tm_year = 2013;

  end_date.tm_hour = 0;  end_date.tm_min = 0;  end_date.tm_sec = 0;
  end_date.tm_mon = 10; end_date.tm_mday = 20; end_date.tm_year = 2013;

  start_time = mktime(&start_date);
  end_time = mktime(&end_date);

  seconds = difftime(end_time, start_time);

  printf ("%.f seconds difference\n", seconds);

  return 0;
}

EDIT : @qchen answer helped lot to solve my problem. one more doubt is there. Below was my update. From the answer

  start_date.tm_hour = 0;  start_date.tm_min = 0;  start_date.tm_sec = 0;
  start_date.tm_mon = 10-1; start_date.tm_mday = 18; start_date.tm_year = 2013-1876;

  end_date.tm_hour = 0;  end_date.tm_min = 0;  end_date.tm_sec = 0;
  end_date.tm_mon = 10-1; end_date.tm_mday = 20; end_date.tm_year = 2013-1876;

tm_year is the year since 1900, then why i getting correct output if i replace 1876 with year between 1876 to 2012.

Answer 1

The problem is that tm_year is the year since 1900, so 2013 would be 113 http://en.cppreference.com/w/cpp/chrono/c/tm

  start_date.tm_hour = 0;  start_date.tm_min = 0;  start_date.tm_sec = 0;
  start_date.tm_mon = 10; start_date.tm_mday = 15; start_date.tm_year = 113;

  end_date.tm_hour = 0;  end_date.tm_min = 0;  end_date.tm_sec = 0;
  end_date.tm_mon = 10; end_date.tm_mday = 20; end_date.tm_year = 113;

Given 2013, mktime will return -1 as the calendar time cannot be represented. You would think that the year 3913 would be a valid calendar time and the reason is related to the year 2038 problem, as pointed out by Joni

Answer 2

OP did not check mktime()` result.

As @Joni mentions, set the tm_isdst field. Use 0 or 1 if you know if how DST is applied, else use '-1' and let the OS make the determination.

@qchen mentioned the year 1900 offset as you likely want .tm_year = 2013-1900.

I assert the underlying issue is using mktime() without checcking if it is (time_t) -1. With robust code, this return value should be tested and missing that opened OP code to unexpected results.

Answer 3

In addition to not specifying the year correctly, you are leaving the tm_isdst field unset. mktime uses this field to determine if the date has or doesn't have daylight-savings time in effect, or if the DST setting should be looked up from timezone databases. This can make the result off by one hour. Add these lines:

/* lookup if DST or not */
start_date.tm_isdst = -1;
end_date.tm_isdst = -1;