C program to display calendar for any year (BGI graphics)

/*
 Program Calender - C program to display calendar for any year (BGI graphics)
 Sandipan Dey
 BCSE, JU, Kolkata
 2002
*/

#include<graphics.h>
#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
#include <time.h>
#include<string.h>

int leapyear(int year) {
    return((year%4==0 && year%100!=0) || (year%100==0 && year%400==0));
}

int firstday(int cday,int cd,int cm,int cy) {
    int td=0, mo=1;
    while(mo < cm) {
        switch(mo) {
            case 2:if(leapyear(cy))td+=29;else td+=28;break;
            case 4: td+=30;break;
            case 6: td+=30;break;
            case 9: td+=30;break;
            case 11:td+=30;break;
            default:td+=31;break;
        }
        ++mo;
    }
    td+=cd;
    if(td%7==1)return(cday);
    else {
        cday+=(1-td%7);
        if(cday>=0)return(cday);
        else return(7+cday);
    }
}

void calender(struct tm* t,int y) {

    int i,j,k=2,cy,cd,cm,change,l,m,n,noday,cday;
    char s[15],s1[15],s2[3];
    int gd=DETECT,gm;
    initgraph(&gd,&gm,"");
    cy=t->tm_year+1900;
    cm=t->tm_mon+1;
    cd=t->tm_mday;
    cday=t->tm_wday;
    cday=firstday(cday,cd,cm,cy);
    if(y>=cy) {
        change=y-cy;
        for(i=cy;i<y;++i)
            if(leapyear(i))change++;
        cday=(cday+change)%7;
    }
    else {
        change=cy-y;
        for(i=y;i<cy;++i)
            if(leapyear(i))
                change++;
    }
    cday=(cday-change)%7;
    if(cday<0)
        cday+=7;
    }
    setcolor(14);
    rectangle(20,10,565,29);
    sprintf(s,"CALENDER %d",y);
    setcolor(12);
    outtextxy(220,15,s);
    setcolor(getmaxcolor());
    for(j=30;j<400;j+=110)
        for(i=20;i<400;i+=185) {
            l=1;
            int m=(i+10)+cday*24;
            n=j+39;
            ++k;
            setcolor(10);
            rectangle(i,j,175+i,100+j);
            setcolor(getmaxcolor());
            switch(k) {
                case 3:sprintf(s1,"  JANUARY");noday=31;break;
                case 4:sprintf(s1," FEBRUARY");if(leapyear(y))noday=29;else noday=28;break;
                case 5:sprintf(s1,"    MARCH");noday=31;break;
                case 6:sprintf(s1,"    APRIL");noday=30;break;
                case 7:sprintf(s1,"      MAY");noday=31;break;
                case 8:sprintf(s1,"     JUNE");noday=30;break;
                case 9:sprintf(s1,"     JULY");noday=31;break;
                case 10:sprintf(s1,"   AUGUST");noday=31;break;
                case 11:sprintf(s1," SEPTEMBER");noday=30;break;
                case 12:sprintf(s1,"  OCTOBER");noday=31;break;
                case 13:sprintf(s1," NOVEMBER");noday=30;break;
                case 14:sprintf(s1," DECEMBER");noday=31;break;
            }
            setcolor(13);
            outtextxy(i+30,j+10,s1);
            setcolor(getmaxcolor());
            setcolor(4);
            outtextxy(i+10,j+27,"S  ");
            setcolor(getmaxcolor());
            outtextxy(i+35,j+27,"M  T  W  T  F  S");
            setcolor(3);
            line(i,25+j,175+i,25+j);
            setcolor(getmaxcolor());
            while(l<=noday) {
                sprintf(s2,"%d",l);
                if(m==i+10)setcolor(4);
                outtextxy(m,n,s2);
                setcolor(getmaxcolor());
                ++l; m+=24;
                if(m>165+i){m=i+10;n+=10;}
            }
            cday=(cday+l--)%7-1;
            if(cday<0)cday+=7;
        }
    getch();
    closegraph();
}

int main(int argc,char* argv[]) {
    time_t timer;
    struct tm *tblock;
    char x[10];
    int y,z;
    timer = time(NULL);
    tblock = localtime(&timer);
    clrscr();
    if(!strcmp(argv[1],"\0"))
        y=tblock->tm_year+1900;
    else
        y=atoi(argv[1]);
    calender(tblock,y);
    getch();
    return(0);
}