Comment 29 for bug 1228360

Don't know if this helps, but I made a small script I named 'fixtime' that restores the clock when it goes away:
=================
#!/bin/sh

apt-get install indicator-datetime
killall unity-panel-service
=================

Just running killall unity-panel-service does not bring back the clock. I have to first install the indicator-datetime (which comes back reporting that it is already the newest version, so nothing actually happens there):

root@IW-UB-01:/# fixtime
Reading package lists... Done
Building dependency tree
Reading state information... Done
indicator-datetime is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
root@IW-UB-01:/#

This brings back my clock. So something with touching the indicator-datetime installation file(s) seems to make a difference.

I hope this helps you guys track the bug down.