[CentOS] Updating hardware clock from cron

Sat Mar 5 08:33:52 UTC 2011
Dr. Ed Morbius <dredmorbius at gmail.com>

on 15:04 Fri 04 Mar, Denniston, Todd A CIV NAVSURFWARCENDIV Crane (todd.denniston at navy.mil) wrote:
> > -----Original Message-----
> > From: centos-bounces at centos.org [mailto:centos-bounces at centos.org] On
> > Behalf Of Kenneth Porter
> > Sent: Friday, March 04, 2011 14:15
> > To: CentOS mailing list
> > Subject: [CentOS] Updating hardware clock from cron
> > 
> > Is there a package to do this?
> > 
> > Normally the hardware clock is set during shutdown if one is running
> > ntpd.
> No, 
> hwclock --systohc is only called at start time (in
> /etc/rc.d/init.d/ntpd), and only if ntpdate got a good time, which is a
> good thing.
> > But if a long-running server shuts down unexpectedly, this isn't
> > done, and the hardware clock might be off by a lot when it comes
> > back up. 
> Not if you are running ntp and it was able to sync, because ntpd
> activates a mode in the kernel that sets the hwclock every 11 minutes
> when ntp declares it got synced.
> If your hwclock is off by a lot when it comes up I believe it is from
> one of the following:
> A) bad cmos battery.
> B) poor cmos clock
> C) confusing info in /etc/adjtime due to using both hwclock --adjust [at
> boot] and ntp (long story, but it is due to both tweaking the clock
> without coordination between them).
> D) booting a different OS with different ideas of timezones.
> E) manual tweaking of time via bios.

F) Hardware clock set to local time and booting after a standard ->
daylight savings or daylight savings -> standard time shift.

I saw this in a large production environmet.

The main effect was that system logs would show a very large slew during
boot, after ntpdate was run.  Annoying, possibly confusing, but not a

Generally, the solution is:

1: use ntp, chrony, or another time service to sync system clocks while

2: set hwclock to UTC, the way Krell intended it to be.

3: periodically sync the hwclock from system time.  I don't really care
if you do this hourly, daily, weekly, or monthly (I'd probably select
daily myself).  But it will tend to avoid big time jumps.

If you're particularly anal, you could periodically compare system and
hwclock time, and raise a flag to replace the CMOS battery when this
starts to drift.  Since other CMOS and BIOS settings can be lost, and
about the only perceptible sign is a drifting hwclock, this is actually
probably a pretty good practice.
Dr. Ed Morbius, Chief Scientist /            |
  Robot Wrangler / Staff Psychologist        | When you seek unlimited power
Krell Power Systems Unlimited                |                  Go to Krell!