曾经在C的时间处理方面卡了较久， 现在特地来做个总结, update@20210807

C time functions demo：

#include        <string.h>
#include        <stdio.h>
#include        <stdlib.h>
#include        <errno.h>
#include        <sys/time.h>
#include        <time.h> /*on linux, this header is also needed*/

int main(){
    struct tm localct ;
    double diff = 0;
    char time_field[100] =  "20190529-22:20:07.227";
    sscanf(time_field, "%04d%02d%02d-%02d:%02d:%02d", &localct.tm_year, &localct.tm_mon, &localct.tm_mday, &localct.tm_hour, &localct.tm_min, &localct.tm_sec);
    fprintf(stderr, "%04d%02d%02d-%02d:%02d:%02d, DST:%d\n", localct.tm_year, localct.tm_mon, localct.tm_mday, localct.tm_hour, localct.tm_min, localct.tm_sec, localct.tm_isdst);
    localct.tm_year = localct.tm_year - 1900;
    localct.tm_mon = localct.tm_mon - 1;
    localct.tm_isdst = -1;
    time_t m_field  = mktime(&localct);
    time_t  now = time(NULL);
    diff = difftime(now, m_field);
    fprintf(stderr, "time diff is %f\n", diff);
   time_t rawtime;
   struct tm *info;
   time( &rawtime );
   info = localtime( &rawtime );
   fprintf(stderr, "Current local time and date: %s, DST=%d", asctime(info), info->tm_isdst);

    return 0;
}

• time_t mktime (struct tm * timeptr); Convert tm structure to time_t

• Don’t forget to adjust members in struct tm (-1, -1900) etc. tm_year is relative to 1900 in POSIX-compliant platforms. Don’t mess up with unix timestamp, see this link

• time_t time(time_t *seconds) returns the time since the Epoch (00:00:00 UTC, January 1, 1970), measured in seconds. If seconds is not NULL, the return value is also stored in variable seconds. see on link.

• double difftime(time_t time1, time_t time2)returns the difference of seconds between time1 and time2 i.e. (time1 - time2). The two times are specified in calendar time, which represents the time elapsed since the Epoch (00:00:00 on January 1, 1970, Coordinated Universal Time (UTC)). see on this

  link.

• struct tm *localtime(const time_t *timer) uses the time pointed by timer to fill a tm structure with the values that represent the corresponding local time. The value of timer is broken up into the structure tm and expressed in the local time zone. see on this link.

time Data Type:

           struct timeval {
               time_t      tv_sec;     /* seconds */
               suseconds_t tv_usec;    /* microseconds */
           };

           struct timezone {
               int tz_minuteswest;     /* minutes west of Greenwich */
               int tz_dsttime;         /* type of DST correction */
           };
		
		  typedef long time_t;
		  
		  struct tm {
               int tm_sec;         /* seconds,  range 0 to 59          */
               int tm_min;         /* minutes, range 0 to 59           */
               int tm_hour;        /* hours, range 0 to 23             */
               int tm_mday;        /* day of the month, range 1 to 31  */
               int tm_mon;         /* month, range 0 to 11             */
               int tm_year;        /* The number of years since 1900   */
               int tm_wday;        /* day of the week, range 0 to 6    */
               int tm_yday;        /* day in the year, range 0 to 365  */
               int tm_isdst;       /* daylight saving time             */	
		};

C/CPP生成时间戳

#include <sys/time.h>
#include <iostream>
#include <chrono>
#include <cstdint>


using namespace std;
using namespace std::chrono;

int main() {
    struct timeval tp;
    gettimeofday(&tp, NULL);
    long int ms = tp.tv_sec * 1000 + tp.tv_usec / 1000; //get current timestamp in milliseconds
    cout << ms << endl;

    uint64_t ms2 = duration_cast<milliseconds>(system_clock::now().time_since_epoch()).count();
    cout <<  ms2 << endl;

}

特别注意

• 不要使用 if(time.second % 60 = 0){event()} 这种去做timer, 十分可能在指定时间时并没有执行到这个判断

• 需要注意sscanf的使用， 格式化读入字符串， 可节省解析字符串的工作

• struct tm localct` 是结构体(可通过.直接访问成员)， 而struct tm *info 是结构体指针(info需要通过->来访问成员)做函数参数时结构体指针会更有效率

• Summer Time Is Daylight Saving Time, there is a tricky field in struct tm called tm_isdst It should be 1 (or a positive value), DST is in effect. It should be 0, because, as you say, DST is not in effect. It should be 0, because DST was not in effect at that time.

  在我们的程序中使用到 struct tm 时必须对tm_isdst成员进行设定， 中国应当为0，不能假定编译器会将其初始化为0， 避免时间差几个小时。