Time and date is not available any more on top bar after I upgraded Ubuntu from 17.10 to 18.04 LTS. Time&Date settings is not available in "Settings", and it's not available in gnome-tweak-tool either(there's no "Top bar" menu in tweak GUI):
Anyone knows how to fix this?
It seems you've logged into a Unity session somehow. If that is not intended, you may want to login to a GNOME session (default in 18.04).
To do that when you boot your system and get to the GDM login screen you should find a cogwheel (⚙️) next to the sign in button. If you click on the cogwheel you should find an Ubuntu (and Ubuntu on Wayland) option. Select it and log in.
