How can I show the date in the gnome 3 menubar?
You can use dconf-editor to change many of the settings (GSettings) on your GNOME desktop environment. For this case you need to install the dconf-tools package. Run dconf-editor and find the appropriate key for your time settings.
e.g. org.gnome.desktop.interface.clock-show-date
[To expand on what liberforce said]
- launch
gnome-tweak-tool, sometimes called Advanced Settings - click the Shell menu, then enable Show date in clock