I installed Freya recently and shortly thereafter the clock disappeared.
Sometimes I can access the calendar pop-up by randomly clicking around where the clock used to be, but this doesn't always work.
I had uninstalled evolution* packages (I thought I didn't need them) and the time indicator dissapeared
I reinstalled it with:
sudo apt-get install --reinstall indicator-datetime
logged out and everything went fine
I have the same issue with a wingpanel clock. I tried all instructions above, but it wasn't helped.
There are processes of the indicator:
So, it should work. However it doesn't:
Can anyone tell me how to fix this bug?
Done! This one works:
sudo apt install --reinstall wingpanel-indicator-datetime