(still about 2018)

Basic elements elements:

The LiPoly battery, permanently powering the RTC and being connected to the Latching circuit and the charger.

The power push button switches on the MCU, which in the first line of the setup, sets a GPIO pin to high, holding the latching circuit "on", the MCU reads the time from the DS3231 over I2C and lights up the LEDs which assemble the corresponding roman numbers.

Whenever the MCU is plugged to an UBS power source, the battery is charged.