gistfile1.txt

jim@x230:~ % sysctl kern.version
kern.version: FreeBSD 11.0-CURRENT #14 r289806: Fri Oct 23 04:55:45 CDT 2015
    root@x230:/usr/obj/usr/src/sys/GENERIC-NODEBUG

jim@x230:~ % cat clock.c
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>

int
main(int argc, char *argv[])
{
	clockid_t id = CLOCK_REALTIME;
	struct timespec ts_now;

	if (clock_gettime(id, &ts_now) < 0) {
		fprintf(stderr, "clock_gettime: %s\n", strerror(errno));
		exit(EXIT_FAILURE);
	}
	system("date");
	system("uptime");
	sleep(2);

	struct timespec ts_then = ts_now;
	ts_then.tv_sec -= 86400 * 16800;
	if (clock_settime(id, &ts_then) < 0) {
		fprintf(stderr, "clock_settime: %s\n", strerror(errno));
		exit(EXIT_FAILURE);
	}
	system("date");
	system("uptime");

	if (clock_settime(id, &ts_now) < 0) {
		fprintf(stderr, "clock_settime: %s\n", strerror(errno));
		exit(EXIT_FAILURE);
	}
	system("date");
	system("uptime");
}
jim@x230:~ % cc clock.c
jim@x230:~ % sudo ./a.out
Sun Nov 22 03:39:23 CST 2015
 3:39AM  up 19 days, 13:52, 8 users, load averages: 0.07, 0.09, 0.03
Sun Nov 23 03:39:23 CST 1969
 3:39AM  up 19 days, 13:52, 8 users, load averages: 0.07, 0.09, 0.03
Sun Nov 22 03:39:23 CST 2015
 3:39AM  up 19 days, 13:52, 8 users, load averages: 0.07, 0.09, 0.03
jim@x230:~ %