Showing the date in Ubuntu 18’s Gnome Panel

The default clock shown in the Gnome Panel has only the time in HH:MM format. If you want to display seconds or include the date, you need an extension called Clock Override. Instructions for installing it are here:

Note, if searching for the extension in Ubuntu Software turns up nothing, go to the “All” tab, select the Add-ons category, click Extension Settings and turn on Shell Extensions. Clock Override will appear in a list of extensions below.