doy2tm.c

#define _XOPEN_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>

time_t doy2tm(const int year, const int dayofyear, struct tm *tm) {

  int leapyear, m, d;

  int d_in_m [] = {31,28,31,30,31,30,31,31,30,31,30,31};

  memset(tm, 0, sizeof(*tm));

  leapyear = (year % 4 == 0) && ((year % 100 != 0) || (year % 400 == 0));

  if( dayofyear < 1 || dayofyear > (leapyear ? 366 : 365) ) {
    return (time_t) -1;
  }

  if(leapyear) { d_in_m[1]=29; }

  for(m=0, d=dayofyear; 
      d>d_in_m[m]; 
      d-=d_in_m[m++]);

  tm->tm_mday=d;
  tm->tm_mon=m;
  tm->tm_year=year - 1900;

  return mktime(tm);
}

int main (int argc, char* argv[]) {
  struct tm tm;
  int y,d;
  char str[128];

  if (argc != 3) {
    fprintf(stderr, "Usage: %s year dayofyear\n", argv[0]);
    return EXIT_FAILURE;
  }

  y = atoi(argv[1]);
  d = atoi(argv[2]);

  if (doy2tm(y,d,&tm) == -1) {
    fputs("doy2tm returned -1\n", stderr);
    return EXIT_FAILURE;
  }

  if (strftime(str, sizeof(str), "%c", &tm) == 0) {
    fputs("strftime returned 0\n", stderr);
    return EXIT_FAILURE;
  }

  printf("%d %d => %s\n", y, d, str);
  return EXIT_SUCCESS;
}