CentOS 7 - how does timedatectl sync time and date without NTP?

Question

I am creating automated tests for an rsync-like utility and I need to be able to set the clock back 24 hours for one of the tests. However, when I use timedatectl to set the time and date they revert back to the current time after 15 seconds.

Checking timedatectl status I see that it is not running NTP:

# timedatectl status
      Local time: Wed 2019-05-15 23:24:17 EDT
  Universal time: Thu 2019-05-16 03:24:17 UTC
        RTC time: Wed 2019-01-16 02:13:40
       Time zone: America/New_York (EDT, -0400)
     NTP enabled: no
NTP synchronized: no
 RTC in local TZ: no
      DST active: yes
 Last DST change: DST began at
                  Sun 2019-03-10 01:59:59 EST
                  Sun 2019-03-10 03:00:00 EDT
 Next DST change: DST ends (the clock jumps one hour backwards) at
                  Sun 2019-11-03 01:59:59 EDT
                  Sun 2019-11-03 01:00:00 EST

I also checked and NTPD is not running and nothing is bound to port 123.

Does timedatectl have another method of syncing time? The CentOS 7 test machine is running in a Docker container under a VirtualBox image so Internet connectivity is somewhat limited. I am also testing on CentOS directly on VirtualBox so I will need to solve for the non-Docker case as well.

You can't set the time separately in a Docker container. It is always the time of the host. Use a fake time library for your application tests. — Michael Hampton, May 16 '19 at 04:41
Thanks Michael. Oddly enough, I am seeing the same behavior with CentOS 7 running directly in VirtualBox and not in Docker so there must be something more to this. — Not a machine, May 16 '19 at 13:11
@Notamachine did you install the VBox Guest Additions. This also includes a time sync with the host. https://www.virtualbox.org/manual/ch09.html#fine-tune-timers — eKKiM, May 16 '19 at 14:48
@eKKiM Yes I did! I looked for 30 minutes in VBox proper but not in the Guest Additions. I will check that now. — Not a machine, May 16 '19 at 16:53
@eKKiM That solved the problem. I disabled time sync in Guest Additions and that solved the problem both for the client and for the Docker image. If you want to submit that as an answer I will mark it as correct. — Not a machine, May 16 '19 at 17:06

score 1 · Accepted Answer · answered May 16 '19 at 17:23

VBox Guest Additions includes and enables time syncronisation with the host by default.

You can disable this with the following command:

vboxmanage setextradata <vmname> "VBoxInternal/Devices/VMMDev/0/Config/GetHostTimeDisabled" "1"

Or add the following line to your yourVM.vbox file

<ExtraDataItem name="VBoxInternal/Devices/VMMDev/0/Config/GetHostTimeDisabled" value="1"/>

score 0 · Answer 2 · answered May 16 '19 at 14:07

timedatectl is called by systemd after specific frequency it checks /etc/adjtime, /usr/share/zoneinfo/, /etc/localtime and many other files to check if any info is available, it having it's own logic to adjust the clock without using ntp.

static int context_write_data_timezone(Context *c) {
    _cleanup_free_ char *p = NULL;
    int r = 0;

    assert(c);

    if (isempty(c->zone)) {
            if (unlink("/etc/localtime") < 0 && errno != ENOENT)
                    r = -errno;

            return r;
    }

    p = strappend("../usr/share/zoneinfo/", c->zone);
    if (!p)
            return log_oom();

    r = symlink_atomic(p, "/etc/localtime");
    if (r < 0)
            return r;

    return 0;

}

Thanks! When I run "systemctl list-units --all | grep time" I do not see timedatectl. Is it embedded in another service or called by another name? — Not a machine, May 16 '19 at 16:52

CentOS 7 - how does timedatectl sync time and date without NTP?

2 Answers2