So you want to build your own watch? The Solder: Time II Watch Kit has you covered. It has a ATmega328P microcontroller, so you can program it using the Arduino software. The kit comes with 4 x 5×7 LED matrix modules and a total of 140 LEDs. You will be able to display time, date, month, words, scrolling messages, and graphics.
You can write games, add sounds, and do a whole lot more with your watch. Costs under $30 at this time.