I don't think that this is possible when you use Gnome screensaver. Since Gnome 2 it has lost almost all its features and now it can only lock screen and blank it (Wikipedia even says that it should be called "screen blanker" rather than screensaver).
You can try XScreenSaver which is more powerful. To install it add the next packages:
xscreensaver-gl. After the installation add it to your "Startup Applications". The command should be
xscreensaver -nosplash. Check this link for the details and the pictures.
man xscreensaver also contains valuable information.
xscreensaver-demo to start configuration window for XScreenSaver.
You have no need to uninstall
gnome-screensaver, just disable it (you can revert all changes back later):
sudo chmod -x /usr/bin/gnome-screensaver. Or rename it to
gnome-screensaver.bak. You can also rename
.bak and make screen lock shortcut work with
xscreensaver with this command:
sudo ln -s /usr/bin/xscreensaver-command /usr/bin/gnome-screensaver-command
Then you can use the standard GLText screensaver, which can display current date/time. Or you can browse https://www.gnome-look.org for some alternatives. Here is nice flip-clock screensaver. There are more screensavers here and here.