输入当前日期，获取下一天的日期

      C可通过time()函数获取当前系统时间，返回的结果是一个time_t类型，其实就是一个大整数，其值表示CUT （Coordinated Universal Time）时间1970年1月1日00:00:00（称为UNIX系统的Epoch时间）到当前时刻的秒数。如下面的函数实现了将Epoch时间到当前的天数以字符的形式存放到四字节的数组中。

#define X 86400  //一天的总秒数
#include <time.h>
#include <stdio.h>

void utctime(unsigned char * p_str)
{
time_t tmpcal_ptr, daytmpcal;

struct tm *tmp_ptr = NULL;

time(&tmpcal_ptr);
daytmpcal = tmpcal_ptr  / X;             //Epoch时间距当前的总天数

p_str[0] = daytmpcal >> 24;
p_str[1] = daytmpcal >> 16;
p_str[2] = daytmpcal >> 8;
p_str[3] = daytmpcal;

return ;
}

      上述程序只能唯一获取当前的时间标志，若需要获取指定某一天的时间，可以给函数增加一个参数days,让daytmpcal+days，但这样的话不同时刻调用该函数，days的取值不同。

      下面的程序实现了已知当前日期，获取下一天（或指定多少天以后）的日期的功能。#include <stdio.h>

unsigned char days_of_month[] = { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
unsigned char days_of_month_sSpecial[] = { 0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

int isSpecialYear(int x) {
if (x % 400 == 0 || (x % 4 == 0 && x % 100 != 0)) {
return 1;
}
return 0;
}

int get_days(int year, int month)
{
if (isSpecialYear(year) == 1)
{
return(days_of_month_sSpecial[month]);
}
else
{
return(days_of_month[month]);
}
}

void get_next_day(unsigned char* p_today, unsigned char* p_tomorrow)
{
//p_today[0] = year-2000; p_today[1] = month, p_today[2] = day;
int year, month, day;
year = (int)(p_today[0]) + 2000;
month = (int)(p_today[1]);
day = (int)(p_today[2]);

day++;

if (day > get_days(year, month))
{
day = 1;
month++;

if (month > 12)
{
month = 1;
year++;
}
}

p_tomorrow[0] = (unsigned char)(year - 2000);
p_tomorrow[1] = (unsigned char)(month);
p_tomorrow[2] = (unsigned char)(day);
p_tomorrow[3] = p_today[3];

}

static unsigned char start_day[4] = {17, 12, 1, 0};

void ndayslater(unsigned char * p_str, int days)
{
unsigned char next_day[4], today_day[4];
memcpy(today_day, start_day, 4);
int i;

for (i = 0; i < days; i++)
{
get_next_day(today_day, next_day);

memcpy(today_day, next_day, 4);
}

memcpy(p_str, today_day, 4);
}