Hardware

The board design was made in Eagle, has been tested and works. It can be freely modified. It can also directly be ordered on the OSHPark website:

DISCLAIMER: I am in no way affiliated with Atmel. I just put their logo on the PCB to show the circuit uses an Atmel chip, which I am a big fan of)

Software

The software was written in Arduino code, is licensed under MIT License, and can be uploaded on the microcontroller via ISP. Be sure to set the desired time in the code prior to uploading, to set the time registers on the RTC.

Functions of the code include:

Hours, minutes and seconds

Day and month

Stopwatch

Temperature reading from the real-time-clock's built-in thermometer (accurate to about 3°C)

To be able to upload the code on the watch, an Arduno IDE board file for an "ATMega328 on a breadboard" running at 8 MHz with the internal crystal, is needed. You can find it here: https://www.arduino.cc/en/Tutorial/ArduinoToBreadboardunder "Minimal Circuit (Eliminating the External Clock)"

Required Libraries