I bought an Olivetti Logos 262 PD from 1980's, whose only guilt was to have a VFD display. I reverse-engineered the keyboard connection and designed a circuit to emulate keypresses with a microcontroller. Once the circuit has been built and tested, I wrote an Arduino firmware that reads an RTC (battery-backed clock) and types the numbers into the calculator. Last but not least, the clock circuitry can be disabled and the calculator used as originally intended.
Display modes are bonded to the way a calculator interacts with humans. Mode is changed towards the end of every full minute and includes:
One more thing needs to be implemented, both in hardware and software: a way to set time and date!
If you need inspiration, the firmware is on github. While I have lots of notes on scrap paper, I do not have a complete schematic diagram to share. If I locate another calculator to convert I will draw a diagram so that it can be reproduced.