Quite possibly built in the late 1930s here's the electro-mechanical digital clock I've just acquired.
It doesn't have a digital display, i.e. one with digits, instead it has 12 lamps for the hours, five for the tens of minutes, and another 10 for the minutes. The lamps are switched by telephone exchange type uniselectors. There are rotary switches on the side to set the alarm.
The calendar part is entirely mechanical, but it has a cam on the month wheel to set the number of days in each month.
I'd love to know if anyone has ever seen this clock or a similar one before, where it was used, who made it, or anything else.
Over the weekend I'll see if I can get it working.
Some nixie clocks have used Strowger (uniselector) switches. See the gallery here http://www.electricstuff.co.uk/nixiegallery.html (search for the Tomlin clock).
Digital (display) clocks did exist in the 1930s - see http://www.scienceandsociety.co.uk/results.asp?image=10305697