Or just power or wind the clock externally. If it's [electro]mechanical, losing power or forgetting to wind it will only extend the period of time for which it

Or just power or wind the clock externally. If it's [electro]mechanical, losing power or forgetting to wind it will only extend the period of time for which it remains closed. Otherwise it would need an internal backup battery, which, if it's only powering an extremely low power clock and nothing mechanical, could last that long with the right chemistry. Solar cells could also last a hundred years, suitably encased, with some reduction in effectiveness due to ionization.