incdate-4.c

#include <stdio.h>

typedef struct
{
  char day, month;
  int year;
}
date;

void set_date (date *d)
{
  d->day = 30;
  d->month = 4;
  d->year = 2012;
}

void inc_date (date *d)
{
  d->day++;
  int days_in_month = 31;  /* FIXME */
  if (d->day > days_in_month)
    {
      d->month++;
      d->day = 1;
      if (d->month > 12)
        {
          d->year++;
          d->month = 1;
        }
    }
}

int main (void)
{
  date today;
  set_date (&today);
  inc_date (&today);
  printf ("%d.%d.%d\n", today.day, today.month, today.year);
  return 0;
}