NavList:

I must say, this has turned out to be an interesting little project.

I have written computer code, so that I can understand the limitations of these Analogue Calendar Calculators.

I have found two things of interest:

The 40 yr circular Calendar (using 7x3=21 segments) can only be fully correct if the Starting Date ends in an "Even" number, So that Ian's at 1994 is OK, but the one of David_P, at 1993 is not. I'm assuming no "typos" are made of the YEARS on the calculator discs.

The 50 yr circular Calendar, (using 7x4=28 segments) does not have any restrictions on the Starting Year. My one shown here, displays ALL of the 50 years correctly.

David and Ian, if you want to quickly check what years are correct on your 40 Yr circular Calendars, I have listed Starting Days for MAY as a postscript to this message. 

I chose **MAY**, because it doesnt involve needing to know the Leap Years:

Because of the way your Circular "Slide Rule" is made David, if the 1st of May DOW (Day of Week) is correct for a certain YEAR, all the other 11 months will be correct for that year.

From the Picture you posted of your 40 Yr span calculator David,

using the table below, we can see that 

2002 Your Calculator is CORRECT

2025 Your Calculator is WRONG 

And using the Picture Ian Gifford posted of his 40 yr span Calculator

2002 is CORRECT

2025 is C0RRECT 

The other issue not mentioned so far, is that EVERY month is shown to have 31 days in it. So one needs to remember what we were taught in School "Thirty Days Hath September" (which has an interesting History).

I'm very grateful to David Pike for raising this topic. 

I had always wondered how my 50 Yr Circular Calendar works, and was it accurate? - now I know it is, the only challenge left, is to try to outlive it !