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.
 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.
 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.