This will allow the panel clock applet to change the time and timezone. Simply enabling it fixes the problem, patch attached. Also, the policykit use flag is currently now a local use flag, we should probably make it global (I will email gentoo-dev about that).
Created attachment 185446 [details, diff] patch to gnome-panel ebuild to enable policykit
gnome-panel-2.26.1 in the overlay has this flag. It will get into portage with 2.26