DO NOT set Date between 9:00pm — 1:00am
DO NOT set Day between 1:00am — 4:30am

The above was taken from the spec document of a Miyota (subsidiary of Citizen) 8205. However, anyone who has owned a watch with a date or weekday mechanism would have at one point (prior to or after breakage) been told not to perform date/day quickset during the hours when they automatically advance.

This is because the date/day driving "finger" would by those times be leaning against the date/day wheel teeth. Should a careless watch owner make use of the quickset function at that point, the "finger" would be subjected to force exerted by the owner's turning of the crown, and thus, either the quickset or the date/day driving wheel would give in and break (or slip, imparting tiny damage that contributes to future breakage).

In Seiko 5606, the quickset would usually give in first as the plastic quickset wheel has a thinner loop around its post compared to the plastic day driving finger, not to mention the metal date driving wheel. This gives rise to the common issue of defective quickset on vintage 5606s.

In Miyota 8205, it's the other way around. The plastic date dial driving wheel would break first when pitted against the metallic calendar correct lever.

Between the design of Seiko 5606 and Miyota 8205, if you ask me, I think 8205 is a better mechanism because the careless owner can manually advance the date/day via quickset, keeping the watch useful until the watch is sent in for repair whereas in the case of Seiko 5606 the date/day would advance but is more difficult to set the date/day without a quickset.

So when a Miyota 8205's plastic date dial driving wheel breaks, it's necessary to source a replacement part, largely available for cheap from China, in order for date/day advancement to become functional again.

I had on my bench, a Citizen 8200A, which is largely the same as a modern Miyota 8205 sans-hacking, whose quickset works but not day/date change past midnight.

Knowing the design of the movement it became immediately apparent that the date dial driving wheel has had it from a careless owner. But I thought let's open it up and see what's the deal before ordering a replacement.

To my surprise, it's (1) metal and (2) not entirely broken.

Well, metal might be a good idea afterall. The bent tail of the date/day finger isn't used to drive either of the date/day wheel, but to be the end of a freely rotating complex shape metal. On the other 2 ends are the date/day wheel driving fingers that actually turn the date/day wheels.

Carefully inspecting the damage, it seems like user had engaged the quickset mechanism past midnight but before the weekday has done changing, causing the weekday driving finger to bend beyond 90° and the tail end, pushing against the wheel below, to bend inward and up, coming into contact with the day dial driving wheel and losing its ability to hold the fingers in place when it's time to change date/day.

I wasn't sure of the proper shape of this tail end. There is no documentation available for that. Does it extend below the bottom of the wheel through the opening? What does the side that's poking out do? Luckily it seemed to be made from soft metal resilient to fatigue from repeated bending, I managed to bend the tail end into a somewhat correct-looking shape.

Most importantly — it works.

So I hope this can be a guide for anyone who's facing the same issue and is looking for the "right" shape to repair this metal date dial driving wheel. But I also wonder, would a modern replacement plastic wheel fit, as the metal one has an entirely different shape and working principle.

Anyway, that's it for this one!

DO NOT set Date between 9:00pm — 1:00am
DO NOT set Day between 1:00am — 4:30am