Use KDE's Digital Clock Widget as a World Clock » Linux Magazine
Display The Clock, RAM And CPU Usage As Circle Widgets On Your GNOME Shell Desktop – Feed Buzzard
Is there any way to remove "Add world clocks" & "Select weather location" from Notification Panel without removing the 'Clock' & 'Weather' app ? : r/ gnome
Flip Clock Widget for Linux - Gnome-look.org
Conky Clock Widget - Gnome-look.org
See World Clocks And Weather Info in the Ubuntu Message Tray - OMG! Ubuntu!