Comment 3 for bug 670772

http://gnome-look.org/content/show.php/Alarm+Clock+Icon+for+light+panel?content=124680 sais:

"Replace "/usr/share/alarmclock/alarm-clock.svg" with this one for the application and/or "usr/share/hicolor/24x24/apps/alarm-clock.svg" with this one (only png works here) for the applet.
Run "sudo gtk-update-icon-cache -f /usr/share/icons/hicolor" and restart the application or remove and re-add the applet."

IMHO best looks Faenza "appointment-soon.png" in /usr/share/icons/Faenza/status/24/ - it also works renamed as svg.