I booted up my Ubuntu 13.10 system running Unity today to find that the clock was missing in the app panel.

Missing Clock App in Ubuntu 13.10

Fortunately this was easy to fix without having to reboot the system. I simply ran the following:

[chris@work ~]$ killall unity-panel-service
[chris@work ~]$ 

The clock instantly appeared.

Ubuntu 13.10 Clock App in Panel

If this doesn’t fix your problem, make sure that you have the indicator-datetime package installed. You can try installing it by running the following:

[gaarai@tenshi ~]$ sudo apt-get install indicator-datetime
The following NEW packages will be installed:
  gnome-control-center-datetime{a} indicator-datetime
0 packages upgraded, 2 newly installed, 0 to remove and 0 not upgraded.
Need to get 0 B/55.7 kB of archives. After unpacking 318 kB will be used.
Do you want to continue? [Y/n/?] y
Selecting previously unselected package indicator-datetime.
(Reading database ... 552412 files and directories currently installed.)
Unpacking indicator-datetime (from .../indicator-datetime_13.10.0+13.10.20131023.2-0ubuntu1_amd64.deb) ...
Selecting previously unselected package gnome-control-center-datetime.
Unpacking gnome-control-center-datetime (from .../gnome-control-center-datetime_13.10.0+13.10.20131023.2-0ubuntu1_amd64.deb) ...
Processing triggers for libglib2.0-0:amd64 ...
Processing triggers for libglib2.0-0:i386 ...
Processing triggers for bamfdaemon ...
Rebuilding /usr/share/applications/bamf-2.index...
Processing triggers for desktop-file-utils ...
Processing triggers for mime-support ...
Processing triggers for gnome-menus ...
Setting up indicator-datetime (13.10.0+13.10.20131023.2-0ubuntu1) ...
Setting up gnome-control-center-datetime (13.10.0+13.10.20131023.2-0ubuntu1) ...
Processing triggers for libc-bin ...

[gaarai@tenshi ~]$ 

Did I help you? Send me a tip.