[chrony-users] What does the reference time really mean?

[Stefan Mayr <stefan@xxxxxxxxxxxxxx>]

Hello everybody,

I'm struggling to understand the output of chronyc. I'm no native 
speaker and initially I understood the reference time is the time we've 
seen last from a source.

But different data from chronyc commands don't add up.

Example:

root@010987c23b86:/# echo "-- date --"; date -u; echo "-- sources --"; 
chronyc selectdata | egrep '^\*|Last'; echo "-- tracking --"; chronyc 
tracking; echo "-- ntpdata --"; chronyc ntpdata 94.16.122.152
-- date --
Sun Sep 15 19:26:57 UTC 2024
-- sources --
S Name/IP Address        Auth COpts EOpts Last Score     Interval  Leap
* 94.16.122.152             N ----- -----  770   1.0   -14ms   +14ms  N
-- tracking --
Reference ID    : 5E107A98 (94.16.122.152)
Stratum         : 3
Ref time (UTC)  : Sun Sep 15 19:10:24 2024
System time     : 1.183240414 seconds fast of NTP time
Last offset     : +0.000603056 seconds
RMS offset      : 0.337173194 seconds
Frequency       : 1.908 ppm slow
Residual freq   : +0.014 ppm
Skew            : 0.306 ppm
Root delay      : 0.025792295 seconds
Root dispersion : 0.002303645 seconds
Update interval : 515.7 seconds
Leap status     : Normal
-- ntpdata --
Remote address  : 94.16.122.152 (5E107A98)
Remote port     : 123
Local address   : 172.17.0.2 (AC110002)
Leap status     : Normal
Version         : 4
Mode            : Server
Stratum         : 2
Poll interval   : 9 (512 seconds)
Precision       : -25 (0.000000030 seconds)
Root delay      : 0.003632 seconds
Root dispersion : 0.000549 seconds
Reference ID    : 11FD0EFD ()
Reference time  : Sun Sep 15 19:03:43 2024
Offset          : -0.000946184 seconds
Peer delay      : 0.022160703 seconds
Peer dispersion : 0.000000142 seconds
Response time   : 0.000024310 seconds
Jitter asymmetry: +0.00
NTP tests       : 111 111 1111
Interleaved     : No
Authenticated   : No
TX timestamping : Kernel
RX timestamping : Kernel
Total TX        : 31
Total RX        : 31
Total valid RX  : 31
Total good RX   : 31

Looking at the current system date and the value from the "Last" column 
I would have thought: it is 19:26:57 and the last packet from the 
selected source was seen 12min 50s ago. So the last expected reference 
time should be 19:14:07

But chrony tracking shows 19:10:24 and ntpdata for this server shows 
19:03:43 as ref or reference time (I guess all these values are UTC). 
Why is none of these values the time I expected and why do those values 
even differ?

In a different environment with Windows servers as sources I've already 
seen chrony tracking showing "ref time"s beeing hours in the past 
although we see NTP packets being received only seconds ago.

Can anyone explain to me what ref time or reference time really means 
with other words than that used in the man pages?

Thank you,

	Stefan

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