Looking for a Timer

I'm looking for a timer with a specific functionality. I want to be able to
set the thing to turn on an electrical appliance at a set time, then turn it
off after a selectible time. I don't want the thing to come on again until
I tell it to. Not one day later or 7 days later.The application is for the
pre-heaters on my RV-6. I've got a battery warmer and a sump heater.

The low-down is that I work 15 minutes from the airport, so I can drop by
very easily at lunch or right after work to set a timer if I think I'm going
to be flying the next day (this is particularly useful for flights early on
Saturday morning). There are many timers that will support this function.
The problem is that most of 'em will then turn on again at the same time the
next day, and the next, and the next. Or maybe, they will turn on a week
later. The situation I want to avoid is where I set the timer for, say,
Saturday morning, then plans go bad and I don't get to the airport for a
week or two and the heaters run on a daily basis or weekly basis until I
reset or unplug the timer.

I've tried the big box stores, Grainger, etc. and have not found a timer
that works the way I want.

Suggestions?

KB