[Windell] of Evil Mad Scientist Laboratories took an ancient Nixie tube based frequency counter and converted it into a clock. The unit he got his hands on is an HP model that was still in great shape. He’s using an internally generated one second pulse as the clock signal, but some modifications are necessary to display time. That’s because the frequency counter is base 10 and clocks use a quirky combination of base 60 and base 12.
It wasn’t too much of a problem to rig up a system to track minutes and seconds. The tens digit for each is monitored by a couple of AND gates that he added to the mix. When they detect a ‘6’ the digit is reset and a pulse increments the next digit as the carry. This is more difficult to accomplish with the hours though. Minutes and seconds count from 0 to 59 but hours don’t start at 0. Instead of over-complicating the logic [Windell] used a bit of slight-of-hand. The Nixie tubes for the hours have been rewired so that when the counter is at 0, the filament in the shape of a 1 lights up. No difference in logic, just a translation that makes them display one digit higher than the actual count.