Since the latest update my clock disappeared.
(I guess through the latest language-plug update)
The workaround from here don't work.
How can I get the clock back to wingpanel?
Try re-installing the date-time indicator
sudo apt-get install --reinstall indicator-datetime
If this doesn't work, try editing the /etc/environment file and put this line at the end of it:
Log out and log back in, and it should display your clock.
For a detailed reading: Launchpad-Bug-Report