[ntp:questions] Win7: ntpd adjusting time backwards

Jeroen Mostert jmostert at xs4all.nl
Sat Dec 8 20:49:07 UTC 2012

On 2012-12-08 21:12, unruh wrote:
> On 2012-12-08, Jeroen Mostert<jmostert at xs4all.nl>  wrote:
>> If my event log is to be believed, ntpd is adjusting the clock to times in the
>> past (with pretty big intervals):
>> As I understand it, it's not supposed to be doing this, instead it should slow
>> the clock down.
> No. ntpd jumps the time, forward or backward, if the time is out by more
> than 128 microseconds.

I think you mean 128 milliseconds. That's what I'm getting from the docs, anyway.

I thought ntp could adjust the time forwards in "big" steps, but would never 
adjust the time backwards. I'm guessing I got confused with the slew-only option 
(-x) for servers that absolutely cannot tolerate the clock going backwards, even 
if it takes forever to adjust it through slew only.

> Note that both operating systems use the coarse adjust at bootup when
> they calibrate the system clock rate. If your timer rate is out by a
> lot, you can recalibrate by hand and readjust the system clock rate
> using that coarse adjust.
Since this machine more or less serves as a guinea pig/model for deployment in a 
network, calibrating stuff by hand is basically not going to be an option in the 
production environment. I'm just going to have to count on that working out.

> The main question is whether or not your jumping indicates more severe
> problems-- ie why is the clock finding itself so far out that it has to
> jump. Why is the usual ntpd fixing mechanism not working properly.
> Do you get this behaviour only at the beginning, or after ntpd has been
> running for days?
At the time of the event, ntpd had been running for 19 hours (same as system 
uptime). This is of course not that long in NTP terms. The first clock 
adjustment (logged in the event log) came at 40 seconds uptime, then 4 hours 
later, and after that it started adjusting about every hour.

With your explanation, I'm perfectly willing to blame crappy network conditions 
and/or crappy pool servers on the frequent adjustments. Looking at another 
machine, I see zero (0) adjustments there since ntpd was configured to use a 
central, local time server (stratum 2). This machine has no time-critical 
services, so as long as it's accurate to within 1 sec/day I can't complain. I 
was just confused about the clock going backwards.


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