The gnome clock applet is not working, as described in http://bugs.gentoo.org/show_bug.cgi?id=304123 The fix described there does NOT work at least on my platform (amd64). I think this might be because of http://bugs.gentoo.org/show_bug.cgi?id=302973 :-(
Works fine for me here on amd64 after rebuilding with revdep-rebuild --library... But you can also try to manually rebuild gnome-panel for ensuring Please provide emerge --info output, also look at your ~/.xsession-errors file since it will contain clock applets errors just after login in your desktop and getting the failure
Manually rebuilding 'gnome-panel' did the trick. ("revdep-rebuild --library libnss3.so.12" does not help though.) I thought, I already tried that, but as "equery belongs '/usr/lib/gnome-panel/libclock-applet.so'" yields no result, I rebuilt 'gnome-applets' instead of 'gnome-panel'... duh. Anyway, thank you for your help!
