Opgaveløsning:
Funktionen tomorrow

#include <stdio.h>
#include <stdlib.h>

#define DATE_MAX 1000

/* Date related types */
enum weekday {sunday, monday, tuesday, wednesday, thursday, 
           friday, saturday};
typedef enum weekday weekday;

struct date {
  weekday day_of_week;
  int day;
  int month;
  int year;
};

typedef struct date date;

/* Funktion prototypes */

int leapYear(int);
int date_before(date, date);
weekday next_day_of(weekday);
void prnt_date(date);
char *name_of_weekday(date);
date tomorrow(date);

int main(void) {
  date calendar[DATE_MAX];
  char ignore;

  date first_date = {tuesday, 17, 11, 2015},
       last_date  = {thursday, 17, 11, 2016},
       current_date;
  int i = 0, j = 0;

  current_date = first_date;
  while (date_before(current_date, last_date)){
    calendar[i] = current_date;
    current_date = tomorrow(current_date);
    i++;
  }

 for (j = 0; j < i; j++){
    if (j % 20 == 0) {
      printf("More? - touch the enter key: ");
      scanf("%c", &ignore);
    }
    prnt_date(calendar[j]);

  }
}  

/* Return the date after d */
date tomorrow(date d){
  date result;
  
  result = d;

  switch(d.month){
    case 1: case 3: case 5: case 7: case 8: case 10: 
      if (d.day < 31)
	++(result.day);
      else {result.day = 1; ++(result.month);}
      break;
    case 4: case 6: case 9: case 11: 
      if (d.day < 30)
	++(result.day);
      else {result.day = 1; ++(result.month);}
      break;
    case 2:
      if (d.day < (leapYear(d.year)? 29 : 28))
        ++(result.day);
      else {result.day = 1; result.month = 3;}
      break;
    case 12:
      if (d.day < 31)
	++(result.day);
      else {result.day = 1; result.month = 1; (++result.year);}
      break;
    default: exit(-1);  break;
  }

  result.day_of_week = next_day_of(d.day_of_week);
  return result;
}

/* Is y a leapyear */
int leapYear(int y){
  int result;

  if (y % 400 == 0) result = 1;
  else if (y % 100 == 0) result = 0;
  else if (y % 4 == 0) result = 1;
  else result = 0;

  return result;
}

/* Is date d1 less than date d2 */
int date_before(date d1, date d2){
 return
  (d1.year < d2.year) ||
  (d1.year == d2.year && d1.month < d2.month) ||
  (d1.year == d2.year && d1.month == d2.month && d1.day < d2.day);
}  

weekday next_day_of(weekday d){
  return (weekday) (((int) d + 1) % 7);
}

/* Print date d */
void prnt_date(date d){
  printf("%9s %2i.%2i.%4i\n", name_of_weekday(d), d.day, d.month, d.year);
}

/* Return the name of the weekday of the date d */
char *name_of_weekday(date d){
  char *result;
  switch (d.day_of_week) {
    case sunday: result = "Sunday";
       break;
    case monday: result = "Monday";
       break;
    case tuesday: result = "Tuesday";
       break;
    case wednesday: result = "Wednesday";
       break;
    case thursday: result = "Thursday";
       break;
    case friday: result = "Friday";
       break;
    case saturday: result = "Saturday";
       break;
  }
  return result;
}