Displayed update timestamps vary by 1 second for the exact same update

[bird21 at sensation.net.au]

Hi all,

If I do several consecutive "show route" queries, the timestamp keeps 
changing back and forth, by about a second. Example:

0.0.0.0/0          via x.x.x.x on em2.100 [aapt 05:20:28] * (100) 
[AS2764i]
0.0.0.0/0          via x.x.x.x on em2.100 [aapt 05:20:28] * (100) 
[AS2764i]
0.0.0.0/0          via x.x.x.x on em2.100 [aapt 05:20:27] * (100) 
[AS2764i]
0.0.0.0/0          via x.x.x.x on em2.100 [aapt 05:20:28] * (100) 
[AS2764i]

I notice (via public route servers) that Quagga and FRRouting seem to do 
the same thing. Example:

       Last update: Thu Nov 25 18:20:31 2021
       Last update: Thu Nov 25 18:20:30 2021
       Last update: Thu Nov 25 18:20:30 2021
       Last update: Thu Nov 25 18:20:31 2021

Does Bird do this in order emulate some convention (like perhaps something 
Cisco has done for decades), or is there some other reason it's displaying 
an inconsistent timestamp?

I have a script that compares the md5 of the current output with the 
previous minute to decide whether to log the routing table, but because of 
this quirk the md5 changes almost every time...

So, is this a bug, or feature? :) And is there some other reliable way to 
determine that Bird's routing table has changed?

Thanks.