System clock is incorrect

Sometimes an update/upgrade will reset some system settings. Try the following as needed:

1 - Reset NVRAM or PRAM on your Mac - Apple Support (for Intel Macs) and Mac startup key combinations - Apple Support (for both types)

2 - boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

You don't need to unplug the Mini each night. Just shutting it down is sufficient. And if you're behind a router you have even more security on or off.

To confer with Old Toad's notes, unplugging the mini is completely unnecessary. That is not any kind of security measure. Powering the mini down is all you need to do to prevent anyone from doing anything to it.

That said, the one thing that is happening when unplugged is the mini can't use a trickle of energy to keep the onboard clock running. Which means the backup battery is dead and can't keep the clock running when it isn't plugged in. Hence the clock resets to the earliest date the firmware is set to recognize.