Don't unplug the power cable then
Ok seriously, the capacitor which provides power to run the clock when the xbox is unplugged is probably stuffed. It can be replaced if it is really a problem (its the 1F capacitor).
Another alternative if the xbox is modded is to setup your dashboard (evo-x or whatever) to connect to a time server on the net and set the clock for you. Of course this means the xbox needs to be on a network with web access.