This is retroactive notes, based on memory of this project

Here is an early test (april 2019) done with the gear changer. At that point I presumed a single axis could hold the gears, but there was too much play up/down so the later design holds the gears with a guide rail. (see Gallery)

Gear calculations

60 minutes, 24 hours, 12 months are all factors of 120. So all gears have 120 teeth, and for a minute 2 teeth is the increment,  5 teeth/hour, 10/month. For weekday it is 17.14 teeth and day is 3.75 teeth. The software works out the nearest tooth. All teeth have to be aligned for the gear shift to work, ie movement is by whole teeth steps. The 1.5° error in positioning a pointer is barely visible. 

However by cheating with the gear module, I can make a gear with a few teeth more or less, and still have it mesh with the servo gear. This will be done RealSoonNow, using a 119 tooth gear on the weekday and 128 teeth n the day gear.

The day scale has no tick mark at 0 as there is no day 0, but there is a 00:00 as time, so esthetically I wanted to avoid placing day 1 there. Otherwise a 124 tooth gear would be used on the day gear. The 28/29/30/31 problem for days in a month. is not a problem as the advancment is under software control it can easily skip from, say, 28 to 1.