By default installation of Gnome desktop environment in GNU/Linux systems does not show the complete date and time in the gnome-panel. You can choose either of the following two options for displaying the date and time in the panel.

[1] Access the gnome-shell extensions site from here. Install Full clock extension. I cannot assure you full guarantee by doing this. Because when I tried, it was a failure, don’t know what happened. You can try this but I like to do via the second method.
[2] Open up the terminal and type the following
user@GNULinux:~$gsettings set org.gnome.shell.clock show-date true
Now date and time will be displayed in a beautiful manner. I prefer this, because it is far better than the first one since it doesn’t require any complex procedures.

Note:- The format in which the date and time displayed is different in both the methods. Through the first one it looks somewhat weird and second one looks pretty. For
example the date 29th June, Saturday 10:33 pm is displayed in two methods as follows,
Method 1–> 22:33:30 2013-06-29 , Saturday
Method 2–> Sat Jun 29, 10:33 PM

What do you think? First or Second?