Using the GUI, the tabbing order seems to be quite strange. I haven't worked out how to select a time zone yet, but I've accidentally stumbled on it twice. It seems to sometimes be 'up' from 'Hide help', but not always.
Having consulted with the GTK+ maintainer this is unfortunately due to how GTK+ tabbin works currently and there is no workaround.
*** Bug 2241285 has been marked as a duplicate of this bug. ***