...a mechanical watch mechanism might be a better bet for the timing - if you can find a power source with sufficient shelf life to wait to be activated.
That's probably the most "practical" idea! I THINK I've heard of mechanical clocks that only have to be wound once a year.
One hundred of these mechanisims with one triggering another in series could probably work. And, for the "final action", probably a bigger spring-driven generator.
You'd have to make sure there's no corrosion and that the lubrication won't dry-out.
There are ways of generating electrcity with thermal differences, etc. But you still need to STORE the energy in a battery (or capacitor) to continuously supply the clock. It's the long-term reliability of the battery (with its nasty corrosive chemistry) that I'm worried about.