In the message log I have several messages of this type:
Cannot read hw.chassis.startup_time: No such file or directory
In these forum, I found several PRs for different platforms, but they are not public.
Also, I see that it is a cosmetic bug, I would like to know which versions have the fix for EX, MX, ACX and QFX platforms.
I was able to find another PR but this one is not public either, however as you mentioned this is a cosmetic issue but as per the PR the issue has been fixed on following versions,
junos:18.2R1 junos:18.3R1 junos:19.4R3 junos:20.1R2 junos:20.2R2 junos:20.3R1
Hope you find this information usefull for your concern.
If this works for you please mark as Solved.