Year - normal, leap or calendar change 1752 year?

year.c

/**
 *  Year - normal, leap or calendar change 1752 year?
 *  Copyright (C) 2011 Matous J. Fialka, <http://mjf.cz/>
 *  Released under the terms of The MIT License
 *
 *  Install as follows:
 *
 *  $ cc -pedantic -ansi -pedantic-errors -Wall -Werror year.c -o year
 *  $ su -c 'cp year /usr/local/bin'
 */

#include <stdlib.h>
#include <stdio.h>

int
january1(const int y)
{
	int d = y + 4 + (y + 3) / 4;

	if(y > 1800)
		d -= (y - 1701) / 100, d += (y - 1601) / 400;
	if(y > 1752)
		d += 3;
	return d % 7;
}

int
main(int argc, char *argv[])
{
	int y = 0;

	if(argc < 2) {
		fprintf(stderr, "Usage: year [year ...]\n");
		return 1;
	}
	while(*++argv) {
		y = atoi(*argv);
		printf("%d is ", y);
		switch ((january1(y + 1) + 7 - january1(y)) % 7) {
		case 1:
			printf("normal");
			break;
		case 2:
			printf("leap");
			break;
		default:
			printf("calendar change");
			break;
		}
		printf(" year\n");
	}
	return 0;
}

Example usage may be as follows:

$ year 1620 1752 2011
1620 is leap year
1752 is calendar change year
2011 is normal year

Enjoy!