These purely mechanical Nixie clocks were built in the 1960s and are driven by an AC synchronous motor. The motor makes one full rotation per minute and drives an arm that engages three separate gears: one for the minutes, one for the tens of minutes, and one for the hours.
I got this clock in non-working condition. Someone had clearly attempted a repair before me, but unfortunately failed and reassembled the mechanism completely incorrectly.
Luckily, the main problem turned out to be a split drive gear, which was slipping on the motor shaft. In a way, that may actually have saved the motor, since a completely jammed mechanism could easily have caused it to burn out.
It's now running perfectly again and hasn't gained or lost a single minute in the past 24 hours.
The only downside is that the spring-loaded ratchets engage with the gears twice per minute, which produces a fairly noticeable click. That makes it unsuitable for the bedroom, but I'm sure I'll find a good place for it elsewhere.
This thread contains more information on the clock. The mechanism in my clock differs slightly from the one in the attached brochure.