Printing a calendar for a given year and month - c

Question

I'm trying to build a program that will print out the month and year that the user inputs. What I have so far ends up with the year having 5 digits and inputting "2020 2" results in 28 days for the month of February.
The code I have so far in this assignment:
#include <stdio.h>
#include<stdlib.h>
int is_leapyear(int y)
{
if ((y % 100 == 0 && y % 100 != 0) || y % 400 == 0)
return 1;
else
return 0;
}
int main()
{
int i, j, year, month;
printf("Please enter the year and month:");
scanf("%d", &year);
scanf("%d", &month);
int a[13] = { 0,31,28,31,30,31,30,31,31,30,31,30,31 };
int la[13] = { 0,31,29,31,30,31,30,31,31,30,31,30,31 };
int c = 1;
int flag = 0;
if (is_leapyear(year))
flag = 1;
for (i = 0; i<month; i++)
{
if (flag == 1)
c += la[i];
else
c += a[i];
}
int s = (year - 1) + ((year - 1) / 4) + ((year - 1) / 400) - ((year - 1) / 100) + c;
int week = s % 7;
printf("Calendar " );
printf("%-d", year);
if (month < 10)
printf("0");
printf(" %d\n", month);
printf("----------------------\n");
printf("Su Mo Tu We Th Fr Sa \n");
printf("----------------------\n");
if (flag == 1) {
for (j = 0; j < week; j++)
{
printf(" ");
}
for (i = 1; i <= la[month]; i++)
{
if ((week + i - 1) % 7 == 0)
{
printf("\n");
}
if (i < 10)
{
printf(" ");
}
printf("%d", i);
printf(" ");
}
}
else {
for (j = 0; j < week; j++)
{
printf(" ");
}
for (i = 1; i <= a[month]; i++)
{
if ((week + i - 1) % 7 == 0)
{
printf("\n");
}
if (i < 10)
{
printf(" ");
}
printf("%d", i);
printf(" ");
}
}
printf("\n");
printf("---------------------\n");
}
The output:
Any help would be greatly appreciated.
Edit: It seems like I was too careless in reviewing my code, minor mistakes observed and problem solved!

Answer 1

The century check on your isleapyear()
if ((y % 100 == 0 && y % 100 != 0) || y % 400 == 0)
is flawed. It checked if the year is divisible by 100 and not divisible by 100, which is a logical impossibility. I think you actually mean
if ((y % 4 == 0 && y % 100 != 0) || y % 400 == 0)
So check if the year is divisible by 4 and not divisible by 100.
As for the 20200, it's because your leading zero for the month is printed before the space is printed. Just change it to
printf("%-d", year);
printf(" "); //put space right away
if (month < 10)
printf("0");
printf("%d\n", month); //no need to print space here
or even better as pointed out by #JonathanLeffler in the comment
printf(" %.2d\n", month);
It deals with space, the leading zero, and the new line in just one call, or just
printf("Calendar %.4d %.2d\n", year, month);
to handle those three parts at once

Answer 2

OP's is_leapyear(int y) fails with various multiples of 4.
Instead:
// Alternative for Gregorian calendar
int is_leapyear(int y) {
if (y % 4 == 0) {
if (y % 100 == 0) { // A century year?
return (y % 400 == 0);
}
return 1;
}
return 0;
}
Deeper: Leap year.