Re: [chrony-users] System Clock is updated before the TAI-UTC offset is applied for PHC source in chrony 6.4.1

[Chris Hodgetts <chris@xxxxxxxxxxxxxxxx>]

Why use the PHC “ 

chronyc tracking
Reference ID    : 50484330 (PHC0)
Stratum         : 1
Ref time (UTC)  : Fri Jan 16 08:40:40 2026
System time     : 0.000000007 seconds fast of NTP time
Last offset     : -0.000000014 seconds
RMS offset      : 0.000000029 seconds
Frequency       : 14.710 ppm slow
Residual freq   : +0.000 ppm
Skew            : 0.017 ppm
Root delay      : 0.000000001 seconds
Root dispersion : 0.000000544 seconds
Update interval : 1.0 seconds
Leap status     : Normal


Because why not…. 
Also I am playing with PTP so it just is what it is and I need the accuracy.

But look at that system time…. It’s in the nanoseconds of accuracy - just because - why not have the best if you can get the best. 


> On 16 Jan 2026, at 19:56, Bill Unruh <unruh@xxxxxxxxxxxxxx> wrote:
> 
> Although this is certainly a pain, I wonder why you are using the PHC source
> at all. Use a couple of network sources plus the gps (yes, gps under chrony
> can get down to sub-micro second accuracy so it would always be the primary
> source, except if it loast tracking, in which case the fallback would be the
> time from the network source, which would be in the microseconds, or 10s of
> microseconds accuracy.
> What accuracy do you need? I know that 1 second in the life of the universe
> would give nice bragging rights, but do you really need that? (and no gps
> would not give that).
> 
> Also, on startup the clock should be assumed to be bad for some time. Why are
> you switching it off at all?If you want accurate time, let the clock and
> chrony run for as long as possible.
> 
> William G. Unruh __|__Hagler Fellow, Distinguished |_Tel:UBC +1(604)822-3273
> Physics&Astronomy _|__ Research Prof, IQSE         |__  US +1(979)7399950
> UBC, Vancouver,BC _|_ TAMU4242, 578 University Dr  |_ unruh@xxxxxxxxxxxxxx
> Canada V6T 1Z1 ____|__College Stn, Tx, USA 77843  _|_www.theory.physics.ubc.ca
> I cannot reply to emails from outlook or hotmail or other microsoft domains.
> 
> On Fri, 16 Jan 2026, Simon Plackett wrote:
> 
>> [CAUTION: Non-UBC Email]
>> 
>> Hi,
>> 
>> Before I send a lot of detail I wondered if anyone had seen this
>> phenomenon. I have just upgraded my local chrony server to Trixie on
>> RPI 5 with chrony 4.6.1.
>> 
>> I had previously been using the ethernet NIC clock as a time source
>> with ptp4l and phc2sys, which worked fine.
>> 
>> During the upgrade I changed to leapseclist from leapsectz as the
>> latter does not work on Trixie.
>> 
>> The issue is that on startup the system clock is set by the PHC source
>> before the TAI-UTC adjustment has been applied which means it jumps
>> 36+ seconds one way and then back again during the startup. This
>> obviously trashes the PHC as a source, and it didn't happen
>> previously.
>> 
>> Essentially PHC is chosen as the source, updates system clock (wrong
>> by ~37s) then the TAI-UTC adjustment is found which causes system time
>> to jump again in the opposite direction by ~37s
>> 
>> I can see this using systemctl status chrony or using journalctl.
>> 
>> I have currently turned PHC off again - but happy to turn back on and
>> send any logs etc that might be useful.
>> 
>> The other source is PPS from GPS, which is even more accurate on the
>> new versions :-)
>> 
>> Thanks
>> Simon
>> 
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