mktime returns wrong timestamp (off by a whole month)

Question

I use mktime to create a unix timestamp from my current local time:

#include <time.h>

int _tmain(int argc, _TCHAR* argv[])
{
  struct tm info;

  // 16.05.2014
  info.tm_mday = 16;
  info.tm_mon = 5;
  info.tm_year = 114; // Years since 1900

  // 08:00:00 Uhr
  info.tm_hour = 8;
  info.tm_min = 0;
  info.tm_sec = 0;

  // Convert to Unix timestamp
  info.tm_isdst = -1;
  time_t timestamp = mktime(&info);
  printf("Timestamp: %i", timestamp);
}

This gives me:

1402898400

When converting this back to a human readable time (via some website) this translates to:

16.06.2014 08:00:00

As you can see this is one month off from what I expected (put in May (5), got out June (6)). Why?

Answer 1

C11 7.27.1 Components of time

int tm_sec; // seconds after the minute — [0, 60]
int tm_min; // minutes after the hour — [0, 59]
int tm_hour; // hours since midnight — [0, 23]
int tm_mday; // day of the month — [1, 31]
int tm_mon; // months since January — [0, 11]
int tm_year; // years since 1900
int tm_wday; // days since Sunday — [0, 6]
int tm_yday; // days since January 1 — [0, 365]
int tm_isdst; // Daylight Saving Time flag

tm_mon starts with 0, not 1. So the value of 5 means it's June, not May.