Manual override. Place it under the lamp and press a button that cuts off battery charger, turning the micro on for a while.
BTW, you still need some kind of supply to keep the RTC running. Put your clock under the lamp and it will display nice blinking 12:00:00, and if you don't charge the battery it will reset as soon as you get it into darkness. (Note the NVSRAM+RTC from TI which otherwise would be great against thisproblem, are rather bulky, so if that's to be a hand watch, they are a bit too big...)


Note solar batteries mostly feed on infrared and wavelengths far exceeding visible spectrum. Most materials we consider "transparent" are quite opaque to most useful solar battery wavelengths - so either leave it exposed or look for a material that doesn't have such a negative impact.