I think the electronics can be challenging (more so from a reliabilty / redundance point of view), but the power sources are more so.
You probably will need to make some assumptions, without spending a lot of money. I would think of a few back-ups, like solar, geo-thermal, mechanism (wind + water), battery + capacitors, etc.
If all the device does is to keep time + go to sleep, its current consumption cannot be that big (ua range).