比较DateTime大小 用C#实现

在C语言中，可以使用time.h头文件中的time_t类型和difftime函数来比较两个DateTime的大小。

具体步骤如下：

1. 将两个DateTime转换为time_t类型，可以使用mktime函数将struct tm类型的时间转换为time_t类型。

2. 使用difftime函数比较两个time_t类型的时间差，如果返回值为正数，则第一个时间较晚；如果返回值为负数，则第二个时间较晚；如果返回值为0，则两个时间相等。

示例代码如下：

#include <stdio.h>
#include <time.h>

int main()
{
    struct tm time1 = {0};
    time1.tm_year = 2021 - 1900;
    time1.tm_mon = 8 - 1;
    time1.tm_mday = 1;
    time1.tm_hour = 12;
    time1.tm_min = 0;
    time1.tm_sec = 0;

    struct tm time2 = {0};
    time2.tm_year = 2021 - 1900;
    time2.tm_mon = 8 - 1;
    time2.tm_mday = 2;
    time2.tm_hour = 12;
    time2.tm_min = 0;
    time2.tm_sec = 0;

    time_t t1 = mktime(&time1);
    time_t t2 = mktime(&time2);

    double diff = difftime(t1, t2);

    if (diff > 0)
    {
        printf("time1 is later than time2\n");
    }
    else if (diff < 0)
    {
        printf("time2 is later than time1\n");
    }
    else
    {
        printf("time1 and time2 are equal\n");
    }

    return 0;
}