I recently upgraded from 18.04 to 20.04. To get my networking icons working I reset my panels (Network icon (and bluetooth icon) does not show up after upgrade from 18.04 to 20.04).
After I reset the panels, the time was missing from the top panel. I tried several fixes, checking settings, etc. but could not get it to work.
Eventually I found I had a missing package, indicator-datetime. All I had to run was:
sudo apt install indicator-datetime
The time did not show up until I rebooted the system.
Note: This is from memory from a few days past; I may have also purged the package and then reinstalled it.