#include <stdio.h>
#include <conio.h>

#define BASE_YEAR 1900

int is_leap(int year);
long date_to_num(int day, int month, int year);
int which_day(int day, int month, int year);
void display_calender(int month, int year);

int month_day[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
char *month_name[] = {"Jan", "Feb", "Mar", "Apr", "May", "Jun",
                  "Jul","Aug", "Sep", "Oct", "Nov", "Dec"};

int main(void)
{
   int i, year;

   for (;;) {
      printf("enter year :");
      scanf("%d", &year);
      if (year < BASE_YEAR)
         printf("year must be grater than %d\n", BASE_YEAR);
      else
         break;
   }

   for (i = 1; i < 13; i++) {
      display_calender(i, year);
      if (i % 3 == 0) {
         printf("press any key to continue");
         getch();
         clrscr();
      }
   }
   return 0;
}
/**************************************************/
int is_leap(year)
{
   return (year % 4 == 0 && year % 100 != 0 || year % 400 == 0);
}
/**************************************************/
long date_to_number(int day, int month, int year)
{
   int i;
   long total = day;

   for (i = BASE_YEAR; i < year; i++)
      total += 365 + is_leap(i);

   for (i = 0; i < month - 1; i++)
      total += month_day[i] + (i == 1 && is_leap(year));
   return total;
}
/**************************************************/
int which_day(int day, int month, int year)
{
   long i = date_to_number(day, month, year);

   return (int) (i % 7);
}
/**************************************************/
void display_calender(int month, int year)
{
   int first_day = which_day(1, month, year);
   int total_day = month_day[month - 1];
   int i;

   if (month - 1 == 1)
      total_day += is_leap(year);
   printf("Sun  Mon  Tue  Wed  Thu  Fri  Sat");
   printf("   %s  %4d\n", month_name[month - 1], year);
   printf("---------------------------------\n");
   for (i = 0; i < first_day; i++)
      printf("     ");
   for (i = 1; i <= total_day; i++) {
        printf("%-5d", i);
      if ((i + first_day) % 7 == 0)
         putchar('\n');
   }
   printf("\n");
}