The original firmware is precise enough to keep the time, but it lacks one feature: keep the time when the clock is powered off.
The present firmware allows the use of a DS3231 RTC module (aka Raspberry Pi RTC).

The XTAL was removed and a 5 pin header was soldered on the board. The fuses were reconfigured to use the internal RC clock that was set to 8MHz. The I2C communication is bit-banged on the former Xtal pins (RA4 and RA5).

After installation:

Note: One of the tactile switches was not working and I have replaced both.

When the RTC does not respond to I2C the clock shows an animation pattern.