I made a digital clock. Originally I planned to use a VFD, but the user interface on a 4 digits was too much of a limitation. (I already have to cramped the UI on my STM8 timer with 16x2 LCD)
I used a STM8S003, 74HC595 and a couple of the cheap green 7 segment displays. The display efficiency is actually not bad as the entire clock runs on 5V at ~20mA.
The RTC is implemented in firmware using a DDS for fine frequency adjustment. A supercap allows the STM8 to work for a few minutes during a power failure. The LED driver is on a separate rail.
The PCB is wired to the original (stripped) PCB to use existing buttons. The same GPIO for the common cathode driver when off-duty is used for polling the buttons.
I could have used the STM8 alone, but the 74HC595 frees up a few GPIO lines. UART, I2C, a Timer pin, power and the debug interface are connected to the headers on an I/O panel PCB using the existing RJ11 cutouts.
(I have since changed the power connector to a MicroUSB as it has much better latching.)
Still working on the user interface and other features.
Side by side view of the old traveler's Casio clock it is replacing as it is much harder to read during night time.