Calendar

#include <stdio.h>
// calendar.c written by martin for 59.102 July 1999
// verson 1.0
int leapyear(int year) {
// is year a leap year?
// the rule for leap years is that a year is a leap year if it
// is divisible by four but not by 100 or it is divisible by 400
  if(((year%4==0) && (year%100!=0)) || (year%400==0))
    return 1;
  else 
    return 0;
}

int daysinmonth(int year, int month) {
// Thirty days hath September, April, June, and November;
// All the rest have thirty-one
// Excepting February alone: Which hath but twenty-eight, in fine,
// Till leap year gives it twenty-nine. 
  int monthlengths[]={31,28,31,30,31,30,31,31,30,31,30,31};
  if ((month==1) && leapyear(year))
    return 29;
  else 
    return monthlengths[month];
}

int printmonth(int year, int month, int firstday) {
  // print a complete month, firstday is
  // the number for the first day of the month (0 is sunday)
  char *monthnames[]={"Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"};
  int i;

printf("\n            %s\n",monthnames[month]);
  printf(" Sun Mon Tue Wed Thu Fri Sat\n");
  for(i=0;i<firstday;i++)
    printf("    ");       // print gaps at start of first week
  for(i=1;i<=daysinmonth(year,month);i++) {
    printf("%4d",i);      // print the day number
    if((i+firstday)%7==0) // new line at the end of a week
      printf("\n");
  }
  printf("\n");
  return (firstday+i-1)%7; // return first day of next month
}

int main() {
  // print a calendar for a given year
  int i;
  int day;
  int year=1989;
 
  // work out what day of the week the first day of the year is.
  // Find the number of days since the start of year 1,
  // this is 365*(year-1) + no of leap years.
  // 1/1/0001 would have been a monday, so add 1.
  day=((year-1)*365+(year-1)/4+(year-1)/400-(year-1)/100+1)%7;
  for(i=0;i<12;i++) 
    day=printmonth(year,i,day); // print each month
  return(0);   // return success code
}