I installed an Intermatic EH40 electronic timer several years ago on a hot water tank at my Parish hall. The timer wires inline to the heater, can be set for up to 7 starts/stops per day, has a manual "on" button, and a battery backup. The time can be set for as little as one minute. It has been in service for several years now, and has worked flawlessly. At the time it cost me about $135.00Cdn, and I'm sure you can find the same product in the U.S.
If you do a search on this site with the key word "hot water timer" you will find the thread. Never mind, found it:
http://ecorenovator.org/forum/conser...ank-timer.html