Unusually for an Arduino, the on-board regulator (MPM3610AGQV inductor-inclusive dc-dc module) is rated up to 21V. The Uno Min data sheet says 6-20V for the board, while the Uno Mini web page ...

The core of the watch is an Arduino Mini. To make it good for time telling, a real-time clock module was added. A DHT11 monitors the temperature and humidity.