I restarted Ubuntu. After the restart and logging in I discover that the Clock in the Panel is missing! I check and see that the package
indicator-datetime is still installed. I open System Settings and go to Time and Date and see that everything in the Clock tab is disabled, so I cannot change it.
This is a bug in Unity as described here. It is supposedly happening due to a configuration value being reset after packages are upgraded.
Anyway, the only solution that works right now is to kill and restart the Panel service:
$ killall unity-panel-service
This worked for me and my clock was back in the Panel! 😊
Tried with: Ubuntu 14.04