The installable extension named Flit that I wrote will show a small time display, and it shows the date if you hover the mouse over it. Flit can also show you a battery state and the system sound level. You can scale the display if you hand-edit Flit's run-time configuration file.
Conky is another installable extension that is used for this purpose in many versions of Linux. Conky is highly configurable, so if you want to tune the appearance in multiple ways. Search for "conky" in the forums here or google it.
Both Flit and Conky are available in the official repository of extensions installable from AppBrowser.