Q: Incorrect Date/Time, SystemServerUI not responding

Hi there, JeffGe.

Your membership in Apple Support Communities is appreciated! It appears that after upgrading your iMac to macOS Sierra, your date and time can be wrong. I'll be glad to help. Be sure to test after every numbered step.

Use Time Machine to back up or restore your Mac -- Before beginning any troubleshooting, I recommend backing up your iMac. It's better to "have" a backup than to "need" a backup.

1. Try safe mode if your Mac doesn‘t finish starting up -- Even though your iMac starts, safe mode will make sure it's starting up correctly.

2. How to test an issue in another user account on your Mac -- This appears to be a software issue. Testing in another user account will isolate this to your user account or determine if it's system wide.

3. About OS X Recovery -- If the issue persists in another user account, reinstall your iMac's operating system.

Have a great day!