/*
   Copyright (C) 2018 Assaf Gordon <address@hidden>
   License: public domain.

   Example of date arithmetics, related to discussion in
   https://lists.gnu.org/archive/html/bug-coreutils/2018-02/msg00002.html

   Explaining why:
     $ date +%Y-%m-%d -d '2003-02-01 - 31 days + 1 month'
     2003-01-29

   Compile with:
     gcc -o date-arith  date-arith.c
*/
#include <string.h>
#include <stdlib.h>
#include <time.h>
#include <stdio.h>

int main()
{
  struct tm a;
  time_t t;
  memset (&a, 0, sizeof a);

  /* Date is "2003-02-01 - 31 days + 1 month" */
  a.tm_year = 103;     /* years since 1900       */
  a.tm_mon  = 1 + 1 ;  /* 0=jan, 1=feb .. 11=dec */
  a.tm_mday = 1 - 31 ; /* day-of-month: 1 to 31  */
  printf("before: %04d-%02d-%02d\n",a.tm_year+1900,a.tm_mon+1,a.tm_mday);

  /* call libc to check and normalize the values in 'struct tm'.
     POSIX specifically says:
     http://pubs.opengroup.org/onlinepubs/7908799/xsh/mktime.html
        "The original values of the tm_wday and tm_yday components of the
         structure are ignored, and the original values of the other
         components are not restricted to the ranges described in
         the <time.h> entry."
     Therefore "tm_mday=-30" is a valid value which will be normalized.

     (error checking ommited for brevity).
  */
  t = mktime (&a);

  /* Now convert back to struct tm */
  localtime_r (&t, &a);

  printf("after:  %04d-%02d-%02d\n",a.tm_year+1900,a.tm_mon+1,a.tm_mday);
  return 0;
}