My iMac showing wrong date and time despite trying everything

If the date & time can't be adjusted automatically or manually, I'd say the likely culprit is some bit of data corruption somewhere in the OS. It's not a bug. A bug would show up among lots of users on many computers. Practically speaking, there's little you can do to prevent this sort of thing. It's just something that might happen from time to time.

I understand that you have the problem solved now, but you might consider reinstalling the OS and see if that properly fixes the problem.

Reinstalling macOS doesn't affect any of your user files or data, it simply lays down a fresh copy of the OS onto the startup drive, replacing any OS files that might be damaged or corrupt. Of course, it's always a great idea to have your stuff backed up before messing with your macOS installation.

The steps for reinstalling macOS include backing up your files. Then you'd reboot the Mac in Recovery and just select the option to Reinstall macOS. That's really all there is to it.

How to reinstall macOS - Apple Support

There is no need to erase, format, partition, delete or otherwise alter the startup drive to do a simple reinstall of the OS.

Are you not aware that the Mac can reach out to Apple's server automatically to get the date and time?

In System Settings > Date & Time you must enable "Set time and date automatically" with the little slider to the right. The source, by default, will be time.apple.com.

I fixed this by running the Maintenance task in Onyx. I got Onyx from, https://titanium-software.fr/en/onyx.html.

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