公元日期计算

adcalc.c

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

int days_m[] = {
	31,
	28,
	31,
	30,
	31,
	30,
	31,
	31,
	30,
	31,
	30,
	31
};

int leap_year(int y) {
	if (y % 400 == 0) return 1;
	if (y % 4 == 0) {
		if (y % 100 != 0) return 0;
		else return 1;
	} else return 0;
}

void calc(int days) {
	int y = 1;
	while (1) {
		int days_y = leap_year(y) ? 366 : 365;
		if (days < days_y) break;
		days -= days_y;
		y += 1;
	}

	int m = 0;	
	days_m[1] = (leap_year(y)) ? 29 : 28;
	for (int i = 0; i < 12; ++i) {
		if (days < days_m[i]) break;
		days -= days_m[i];
		m += 1;
	}
	
	printf("y=%d m=%d d=%d\n", y, m, days);
}

int main(int argc, const char* argv[]) {
	if (argc <= 1) return -1;

	int days = atoi(argv[1]);
	calc(days);

	return 0;
}

这是不对的，因为历法在中间发生过一次变化，从儒略历变成格里历。