Time missing from panel

If you're running Unity, not Gnome Shell, my guess is that indicator-datetime isn't running. Try running /usr/lib/indicator-datetime/indicator-datetime-service from a terminal window. Hopefully that'll fix things. If not, please edit your question to include the exact error message(s) you received.

To launch a terminal, hit Ctrl+Alt+T. You can also type this command to get a list of indicators that are currently running:

ps -ef | grep [i]ndicator

To get it back if it is missing or accidentally uninstalled just add it:

sudo apt-get install indicator-datetime 

In Ubuntu 13.10, run /usr/lib/x86_64-linux-gnu/indicator-datetime-service in terminal can be a solution.

TIPS: Use ps -ef | grep [i]ndicator to find the indicators' location.

This did it for me:

pkill -f datetime

Edit: Kills the process that displays time. Ubuntu will probably then restart the process.