Calender (for all years)

/*
ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ
ÍÍÍ»
º                     CALENDAR
ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ
ÍÍͼ*/

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

static char *month[] = {
            "",
            "Jan",
            "Feb",
            "Mar",
            "Apr",
            "May",
            "Jun",
            "Jul",
            "Aug",
            "Sep",
            "Oct",
            "Nov",
            "Dec",
            };
static char *weekday[]={
            "Sun",
            "Mon",
            "Tue",
            "Wed",
            "Thu",
            "Fri",
            "Sat",
            };

main()
{
int mm, yy;
int key;
clrscr();
printf(
"


        ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»");
 printf(   "
        º                                            º");
  printf(   "
        º                   Calender                   º");
  printf(   "
        º                                             º");
  printf(
"
        ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
");
printf("

****Press any key****");
getch();
clrscr();
printf("

ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»");
  printf("
ºCalender from 01-01-01  to infinity....      º");
  printf("
ºEnter In this format: MM YYYY                  º");
  printf("
ºWriten in C language..                           º");
  printf("
ºDesign by: K.E. Joseph                  º");
  printf("
ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
");
l1:
printf("
Please Enter month & year :");
scanf("%d %d", &mm, &yy);
if(mm < 1 || yy<1)
{
 printf("
 Enter Correct data.. ");
 goto l1;
}
while(1)
{
    clrscr();
    display_calendar(mm, yy);
    puts("For navigation, use the arrow keys.");
    puts("To enter another date, press the Insert key.");
    puts("For RESET press INSERT key");
    puts("Press ESC to exit.");
    printf("
ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»");
    printf("
º           Design by :K.Ebenezer Joseph      º");
    printf("
ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
");
    key = getkey();
    switch(key)
    {
    case 72://up arrow key
        yy++;
        break;
    case 80://down arrow key
        yy--;
        break;
    case 75://left arrow key
        if(mm==1)
        {
            yy--;
            mm=12;
        }
        else
            mm--;
        break;
    case 77://right arrow key
        mm++;
        if(mm>12)
        {
            yy++;
            mm%=12;
        }
        if(mm==0)
            mm=12;
        break;
    case 1://escape key
        exit(0);
    case 82://Insert key
        return main();
    }
}
}


display_calendar(int mm, int yy)
{
    char i, days;
    char first_day;
    char lines=1;

    first_day=find_day(1, mm, yy);
    printf("
ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»");
    printf("
º           %s-%d                            º", month[mm],yy);
    printf("
ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
");
    for(i=0; i<7;i++)
        printf("%s    ", weekday[i]);
    puts("");
    for(i=0; i<first_day;i++)
        printf("    ");

    //Find total no. of days in the month
    if (mm!=2)
        switch(mm)
        {
            case 4:
            case 6:
            case 9:
            case 11:
                days=30;
                break;
            default:
                days=31;
        }
    else
        isleapyear(yy)?(days=29):(days=28);

    for(i=1;i<=days;i++)
    {
        printf("%d    ", i);
        if((i+first_day)%7==0)
        {
            puts("");
            lines++;
        }
    }
    puts("
");
    switch(lines)
    {
        case 4:
            puts("");
        case 5:
            puts("");
    }
}

find_day(int dd, int mm, int yy)
{
    int odddays, leapyear;

    leapyear=isleapyear(yy);

    odddays=odddayyear(yy)+odddaymonth(mm,leapyear)+odddayday(dd);
    odddays %= 7;
    return odddays;
}


/*Every 4th year is a leap year, however centuries are not leap years
except those divisible by 400
for example:
1990 is NOT a leap year
2000 is a leap year
2004 is a leap year
*/
isleapyear(int yy)
{
    if(yy%4==0 && (yy%100!=0 || yy%400==0))
        return 1;
    else
        return 0;
}



//Calculates odd days upto first day of year

odddayyear(int yy)
{
    int odddays=0;
    yy-=1;
    if(yy>=400)
        yy%=400;
    if(yy>=100)
    {
        odddays=yy/100;
        odddays *= 5;
        yy%=100;
    }

    odddays += yy;
    odddays += yy/4;

    return odddays;
}



odddaymonth(int mm, char leapyear)
{
    switch(mm)
    {
        case 1:
            return 0;

        case 2:
            return 3;

        case 3:
        case 11:
            return leapyear?4:3;

        case 4:
        case 7:
            return leapyear?0:6;

        case 5:
            return leapyear?2:1;

        case 6:
            return leapyear?5:4;

        case 8:
            return leapyear?3:2;

        case 9:
        case 12:
            return leapyear?6:5;

        case 10:
            return leapyear?1:0;
      }
}

odddayday(int dd)
{
    return dd%7;
}


//Returns scan code of the key that has been hit
#include "dos.h"
getkey()
{
    union REGS i, o;

    while (!kbhit())
        ;
    i.h.ah=0;
    int86(22, &i, &o);
    return (o.h.ah);
}


/*******************H I N T S *********************
Every 4th year is a leap year,
however centuries are not leap years except those divisible by 400
for example:
1990 is NOT a leap year
2000 is a leap year
2004 is a leap year

for example:
-------------

1 ordinary year = 365 days = (52 weeks + 1 day) = 1 odd day
        that's why if this year 23th August is on Tuesday,
it will be on Wednesday next year

1 leap year      = 366 days = (52 weeks + 2 days) = 2 odd days
100 years        =        76 ordinary years     +        24 leap years
           = [(76*52) weeks+76 days] + [(24*52) weeks+48 days]
           = 5200 weeks + 124 days
           = 5217 weeks + 5 days
           = 5 odd days
200 years        = 10 odd days = 3 odd days
300 years        = 15 odd days = 1 odd days
400 years        = 20 + 1 odd days = 0 days

800 years        = 0 odd days

Sunday is the 0th odd day, Monday the 1st odd day and so on*/