Here’s the thing abou the sun: it won’t go out for a very, very long time. Fog burns off. Clouds blow away. The earth will turn, night will fade away, and dawn will arrive. Every day. So a solar powered wristwatch just might work! Here’s more from Serhii Trush, via hackaday:
The wristwatch’s operation is demonstrated in the linked video (in Ukrainian, auto-generated subtitles available): to read out the current time, the button in the center is pressed, which first shows the hour, then the minutes (in 5 minute intervals).
After this the ATtiny13 MCU goes back to sleep, briefly waking up every 0.5 seconds to update the time, which explains why there’s no RTC crystal. The 12 BPW34S photodiodes are enough to provide 2 mA at 0.5 V in full sunlight, which together keep the LIR2430 cell charged via a Zener diode.