This is a great ATtiny85 watch that uses a SSD1306 OLED display and is powered by a CR1220 battery.
Today core subject, ATtiny85. Someone suggest me to use SOIC version to reduce the watch size and I found the coin cell mAh calculation method will count the battery voltage down to 2.0 V, so this watch require a low voltage version MCU to keep it stable. So I have ordered an ATTINY85V-10SU. (much expensive $_$)
ATtiny85 Ring Watch – [Link]