strptime() reads years relative to 1900AD

Question

I'm using a function that I've understood is not part of the standard C library. I found information about it here: 21.4.6.1 Interpret string according to given format.

It is a very helpful function and does exactly what I need. However, when using it in this very simple way:

#include <stdio.h>
#include <time.h>

int main()
{
    struct tm timedata;
    strptime("1/1/1", "%d/%m/%Y", &timedata);
    printf("day: %d\nmonth: %d\nyear: %d\n", 
        timedata.tm_mday, timedata.tm_mon, timedata.tm_year);
}

strange data is placed in the struct tm timedata. Output is:

day: 1
month: 0
year: -1899

Which doesn't make much sense to me. 1/1/1 should result in:

day: 1
month: 1
year: 1

From what I can see, this behaviour isn't to be expected when reading the GNU manual. For example:

%Y The year as a decimal number, using the Gregorian calendar.

Last time I checked, the Gregorian calendar didn't start at 1900A.D.

EDIT:

Rewrote code to check return value of function. Same problem persists.

#include <stdio.h>
#include <time.h>

int main()
{
    struct tm timedata;
    if(strptime("1/1/1", "%d/%m/%Y", &timedata))
    {
        printf("day: %d\nmonth: %d\nyear: %d\n", 
            timedata.tm_mday, timedata.tm_mon, timedata.tm_year);
    }else{
        printf("Error");
    }
 }

Output:

day: 1
month: 0
year: -1899

Answer 1

Code works correctly. struct tm does not use the same offsets as OP thought. @underscore_d

// C11dr §7.27.1 4
int tm_mon; // months since January — [0, 11]
int tm_year; // years since 1900

---

int main() {
    struct tm timedata;
    char *p = strptime("1/1/1", "%d/%m/%Y", &timedata);
    if (p) {
      printf("day: %d\nmonth since January: %d\nyear since 1900: %d\n",
        timedata.tm_mday, timedata.tm_mon, timedata.tm_year);
      printf("day: %d\nmonth: %d\nyear: %d\n", 
        timedata.tm_mday, timedata.tm_mon + 1, timedata.tm_year + 1900);
    } else {
      puts("Failed");
    }
}

Output

day: 1
month since January: 0
year since 1900: -1899
day: 1
month: 1
year: 1

---

Note: strptime() is defined in POSIX.1-2001 and POSIX.1-2008, also documented in POSIXy systems using man 3 strptime. It is not in the standard C library.