请问用c语言，知道当前系统时间，求6小时之前的时间，该如何就算

请教用c语言，知道当前系统时间，求6小时之前的时间，该怎么就算？

在MFC下，我知道有CTimeSpan类可以实现，但是在c语言下该怎么实现呢？

c语言的函数 difftime 只是计算两个时间的差，现在就是要知道时间间隔6个小时，计算另外一个时间该怎么计算？？

[解决办法]
time_t t = time()-6*3600;

asctime( localtime(&t) );
[解决办法]

C/C++ code

#include <time.h>#include <stdio.h>int main(void){    time_t now, before6hr;    struct tm *nowTm, *before6HrTm;        now = time((time_t)NULL);    before6hr = now - 3600 * 6L;    nowTm = localtime(&now);    printf("Now is\n%d.%d.%d %d:%d:%d\n", nowTm->tm_year + 1900, nowTm->tm_mon + 1, nowTm->tm_mday, nowTm->tm_hour, nowTm->tm_min, nowTm->tm_sec);    before6HrTm = localtime(&before6hr);    printf("Six hours before is\n%d.%d.%d %d:%d:%d", before6HrTm->tm_year + 1900, before6HrTm->tm_mon + 1, before6HrTm->tm_mday, before6HrTm->tm_hour, before6HrTm->tm_min, before6HrTm->tm_sec);}
[解决办法]
探讨

C/C++ code#include <time.h>
#include <stdio.h>

int main(void)
{
    time_t now, before6hr;
    struct tm *nowTm, *before6HrTm;
    
    now = time((time_t)NULL);
    before6hr = now - 3600 * 6L;
    nowTm = localtime(&now);
    printf("Now is\n%d.%d.%d %d:%d:%d\n", nowTm->tm_year + 1900, nowTm->tm_mon + 1, nowTm->tm_mday, nowTm->tm_hour, nowTm->tm_min, nowTm->tm_sec);
    before6HrT…