You have 2 clocks to choose from.
One is shown in the "indicator applet complete" as seen in Unity and one is the old style date time.
The "indicator applet complete" and "clock" applets can be added to the panel with alt+Super+Rclick on the panel.
Same shortcut on an applet to move or remove.
You appear to already have the "indicator applet complete" on the panel but not showing date/tlme.
In terminal check the dconf setting
Code:
gsettings get com.canonical.indicator.datetime show-clock
If the value returned is "false", reset the key to the default value of "true"
Code:
gsettings reset com.canonical.indicator.datetime show-clock
Prefer the other clock and don't want to show this one, run...
Code:
gsettings set com.canonical.indicator.datetime show-clock false