Missing date & time applet from top Unity panel
Solution 1:
Having date and time gone on Ubuntu 13.10 beta, after next reboot, this command solves the problem:
killall unity-panel-service
Solution 2:
Reinstall indicator-datetime. It should be installed by default, but just in case you have removed it unknowingly, it is best to run the install command again.
sudo apt-get install indicator-datetime
Next, we are going to reconfigure the date time:
sudo dpkg-reconfigure --frontend noninteractive tzdata
Lastly, restart unity.
sudo killall unity-panel-service