time_t time (time_t* timer);
输出自Unix纪元即1970年1月1日0点0分0秒（国际标准时间）以来的秒数。
示例time.cpp

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

int main(int argc, char** argv)
{
	time_t ticks;
	char buff[1024];
	ticks = time(NULL); //or time(&ticks);
	snprintf(buff, sizeof(buff), "%.24s\r\n", ctime(&ticks));
	printf("%.10ld\r\n", ticks);
	printf("%.24s\r\n", buff);
	return 0;
}

编译

gcc time.cpp -o time

结果
官方示例中还有一些新玩法：https://www.cplusplus.com/reference/ctime/time/?kw=time

#include <stdio.h>      /* printf */
#include <time.h>       /* time_t, struct tm, difftime, time, mktime */

int main ()
{
  time_t timer;
  struct tm y2k = {0};
  double seconds;

  y2k.tm_hour = 0;   y2k.tm_min = 0; y2k.tm_sec = 0;
  y2k.tm_year = 100; y2k.tm_mon = 0; y2k.tm_mday = 1;

  time(&timer);  /* get current time; same as: timer = time(NULL)  */

  seconds = difftime(timer,mktime(&y2k));

  printf ("%.f seconds since January 1, 2000 in the current timezone", seconds);

  return 0;
}

结果

414086872 seconds since January 1, 2000 in the current timezone