The only cert I have in my Certificates panel is freenas-pre-certui. It was generated with a date older than the hardware its on, which has never had a bad clock set in the time I've owned it, and older than the time I've been using FreeNAS:
Valid dates: Wed Sep 7 01:02:33 2011 to Fri Oct 7 01:02:33 2011
"Certificate 'freenas-pre-certui' is expiring within -3112 days.
Tue, 25 Feb 2020 07:19:34 PM (Europe/London)"
It's not clear to me whether the badly-dated cert was generated then, or whether FreeNAS first decided to start alerting on it then. As above, either way the date is long before any possible sane date so the generation script needs fixing.
I update to new releases pretty much as soon as they go live, which may help with version timelines.
A recent bug on this https://jira.ixsystems.com/browse/NAS-105669 was I think erroneously closed as dupe of https://jira.ixsystems.com/browse/NAS-105664 . This is not a cert that's been replaced, it was created by the system/installation/migration (no idea which) with a bad date range and there's no info on whether it's important or not.