C语言计算日期间隔天数的经典算法解析

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#include <stdio.h>
#include <stdlib.h>
 
int day_diff(int year_start, int month_start, int day_start
			, int year_end, int month_end, int day_end)
{
	int y2, m2, d2;
	int y1, m1, d1;
	
	m1 = (month_start + 9) % 12;
	y1 = year_start - m1/10;
	d1 = 365*y1 + y1/4 - y1/100 + y1/400 + (m1*306 + 5)/10 + (day_start - 1);
 
	m2 = (month_end + 9) % 12;
	y2 = year_end - m2/10;
	d2 = 365*y2 + y2/4 - y2/100 + y2/400 + (m2*306 + 5)/10 + (day_end - 1);
	
	return (d2 - d1);
}
 
int main(void)
{
	printf("%d\n", day_diff(2015, 1, 1, 2015, 1, 8));
	printf("%d\n", day_diff(2015, 1, 29, 2015, 2, 9));
	
	return 0;
}