Add the date to Gnome 3 menubar (gnome-tweak-tool or dconf-editor)

New Way

Install gnome-tweak-tool

dnf install gnome-tweak-tool

In the Top Bar section, enable show date, show seconds. Restart gnome (logout, login).


Old Way

Install dconf-editor
In Fedora 23 its using DNF,

dnf install dconf-editor

navigate to org.gnome.desktop.interface

Enable:

clock-show-date
clock-show-seconds
clock-format