How to coordinate the clock when dual-booting with Windows

Duane Whitty duane at
Mon Dec 11 23:15:42 UTC 2017

On 17-12-11 06:48 PM, Kevin O'Gorman wrote:
> On Mon, Dec 11, 2017 at 1:07 PM, Drew Einhorn <drew.einhorn at
> <mailto:drew.einhorn at>> wrote:
>     This is interesting, but I'm uncertain how to use it.  I found the
>     list of servers in the registry.  I didn't put them there, but there
>     are 5, and number 5 appears as the default.  It is
>     <> which I sure hope is not unreliable. 
>     However, I still get Windows showing the time off by 8 hours when it
>     boots up, and I'm supposing that's because it is using the time in
>     the BIOS DRAM, but I'm unsure because as near as I can tell the BIOS
>     is set to local time; at least that's what I see when I'm in BIOS
>     setup.  Frankly I'm all confused.
>     It would be nice if Windows would use <> and
>     apply the timezone, ignoring BIOS completely.
>     -- 
>     Kevin O'Gorman
>     #define QUESTION ((bb) || (!bb))   /* Shakespeare */
>     	Please consider the environment before printing this email.
>     NTP is probably assuming system clock is UTC and correcting based on
>     your time zone settings. This way ntp can assume all clocks are UTC
>     and not have figure out all the relevant time zone corrections.
>     Does this make sense for your time zone?
>     Remote users can set their time zone environment variables for their
>     local time zone. Things work out nicely on Unix or Linux based systems.
>     I don't know If there is a right way to configure Windows so it
>     knows the hardware clock is UTC and the user time zone is different.
>     If not, you have different issues depending on whether the hardware
>     clock is UTC or local time. You may have to decide which issues are
>     the bigger problem.
>     Windows may screw things up for spring and fall daylight savings
>     adjustments, if Windows thinks the hardware is on local time when
>     it's really on UTC.
>     If the latest updates are not installed, your systems may change to
>     or from daylight savings on the wrong weekend. This can be an issue
>     for all operating systems.
>     My daughter's alarm clock switched itself from daylight savings time
>     to standard time on the wrong weekend this year. And, there is no
>     way to update the rules for when to change.
> I reorganized this reply.  Please bottom-post on the list.
> What you say sounds good, but it's a bit confusing to me.  Both Windows
> and Linux are NTP-capable, so when you write "NTP is ...." you don't say
> which one you mean.  But I take it you're writing about Linux NTP, and
> what you wrote agrees with what I think I knew about Linux.  I am not
> aware of a way to configure Windows to do what Linux does, but there's
> been some talk about making Linux do what Windows does instead.  I
> thought I had done it but it didn't work, and now I have a situation I
> don't understand at all.  Follow along with me....
> I'm running Linux and Linux thinks it's 2 PM on Dec 11 PST.  Which it is.
> I reboot and enter BIOS setup.  BIOS thinks it's 2 PM on Dec 11, which
> it still is in this time zone but AFAIK BIOS has no concept of time
> zones.  Maybe my fix worked and linux is storing local time.  But wait,
> let's check.
> I boot into Windows.  Windows thinks it's 6 AM on Dec 11, which is 8
> hours off.  I didn't think Windows could do that, as I wrote above.
> So I fix it.  Windows now thinks it's 2 PM on Dec 11.  I reboot.
> In BIOS, the time is 22:xx (10 PM). off by 8 hours.  So Windows changed
> the BIOS time either when I changed the time or when it shut down, or
> both, and set it to UTC based on Windows' knowledge that we're in the
> UTC-8:00 zone.
> Linux comes up and thinks it's 2 PM on Dec 11 PST.  Which it is.  But
> this is weird.  I was expecting it to accept the BIOS time of 10 PM as
> being local time.
> On a hunch, I ran ntpdate, and here's a new piece of the puzzle:
> 11 Dec 14:37:01 ntpdate[6074]: no servers can be used, exiting
> So now I think I should be configuring NTP.
> /etc/ntp.conf contains a few lines that look like NTP servers.  They
> must have come with the distro:
> # Use servers from the NTP Pool Project. Approved by Ubuntu Technical Board
> # on 2011-02-08 (LP: #104525). See for
> # more information.
> pool <> iburst
> pool <> iburst
> pool <> iburst
> pool <> iburst
> # Use Ubuntu's ntp server as a fallback.
> pool <>
> So why are no servers usable?  Is it possible my router is blocking
> NTP?  Is it possible the standard config file is bad?  This is Ubuntu
> 16.04.3.
> Clues, anyone?
> -- 
> Kevin O'Gorman
> #define QUESTION ((bb) || (!bb))   /* Shakespeare */
> 	Please consider the environment before printing this email.
I'm pretty sure Windows has ever only supported setting the hardware
clock to local time. Also, doesn't Microsoft run some time servers that
Windows systems should use?  If so I wouldn't connect to NTP servers
that expect UTC.

Could hwclock (8) work?  I.e., manually set the bios/hardware clock to
local time which Windows uses/expects and use #hwclock -w to set the
system time from the hardware time on your Linux system.

Best Regards,

Duane Whitty
duane at

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