This is only my guess at your brief description since I wasn't there to see exactly what you did either intentionally or accidentally:

- Date wheel: two items need to be engaged. A flat "hook" and a "plastic wheel" under the date wheel. The series of small holes on the date wheel allows you to inspect whether both are engaged. The correct procedure would've been to test this before installing the dial. Maybe the date wheel was disturbed and the flat "hook" became disengaged.

- Don't force turning the stem. This may be a more serious problem. There's a chance the minute wheel came out of position while you had the watch disassembled. Force-turn a minute wheel can strip the teeth.

I don't have a 82xx in front of me, but I think the stem is removed when it's in the fully pushed in position. It should've been release by pressing that button on the movement.

- "Keyless works" is the mechanism that keeps the stem in and allows you to click through the multiple positions. (forced?) Removal/insertion of the stem when it's in the wrong position could have messed it up.

Since you've already ordered a new movement, might as well learn with this movement: take it apart, reassemble, repeat. That's how many of us learned

If you're using the linked video, the mistake he made is that he didn't immediately reinsert the stem after taking the movement out of the case. The correct procedure would've been to keep the stem in the movement untill latter part of disassembly.

From:

 

Thank You so Much for Your Rapid and Detailed Response !

I will try to take the movement apart and see what kind of damage I've done to it :/

In the Meantime, I've noticed also the Second Hand (and I guess the whole movement) Stopping at around 9:50PM, just when the Date is about to Change, I think there's more that has been engaged in a wrong way by my unprofessional handling.

This is how it looks just when I took out the Plate on the Date Side. Didn't get any further as I was overwhelmed a bit

Thanks Again ! [emoji4]

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I apologize, I previously misunderstood about the "date" wheel; I thought you meant "day."

First thing: Do you still have that small round flat spring under ths plate?

Can't see anything wrong from the photos. Since you got this far, I would suggest this:

From the way the movement is in the photo: Install the stem while pressing the release button. Observe closely to see how it fits. Click through all the positions. Test each function to observe what's stuck. If it all appears to work, start reassembling and test after every step.

Check: is the stem fully inserted through the "clutch wheel"? Is the "setting lever spring" in the correct groove on the stem?

If the problem continues and cause not obvious, keep the stem in and go to the next part.

Keep disassembling the calendar side but keep that other side of the movement completely intact. Leave the keyless works in place as-is (i.e. don't remove the screws for the "setting lever spring" or the "yoke"). Don't try to remove the cannon pinion.

When that's the done, remove and reinstall the stem again to test if it would click through all the setting; if anything is wrong, you should be able to see it.

Then reassemble one component at a time. Test as you go and observe how each components works with other parts. Keep everything perfectly clean but don't use any type of fluid or lube.

If anything is stuck or feels stiff, back off immediately; try to discover what's wrong.

Here's a video for Seiko 7S26. Although it's different than 8215, you can see how the calendar side is assembled and how to test after each step.

There's a good chance you can recover from this. Take a photo before you remove each component, this is how not to get lost.

Keep the stem installed and fully pushed in while you work on the watch. Remove it only when you need to place the movement back into the case.

If you haven't already done so, I encourage you to get the Miyota 8215 manual: https://www.cousinsuk.com/document/search?SearchString=miyota+8215

 

p.s. I noticed in you mentioned "9:50pm." The correct way to do this is advance the watch until just after the date finished changing to the next. Then install the hands for midnight. So date will always finish advancing at midnight.

Don't manually change the date between 10pm and 1am, that's how the mechanism could get messed up.

 

Allright, the New Movement has Arrived and Thanks to You changing it was a real pleasure, the 8215 Stem Removed very smoothly in Time Setting Position and came back to place several times during the Change the same Way

Turned the Crown just until the Date has Changed and then attached the Hands at 12 o'clock (Thanks for that !)

I also have discovered through some research that all this Time I've had the Movement Holder Clamps in the Opposite Way [emoji51] Fixed that one .

I will be Trying to Figure out what went wrong with the old movement and post some pictures in Here so that someone else could maybe find it helpful too.

Thank You again !

Here's some pictures if You're Interested, It might be a strange "Design", but I personally Love It, as this is my very first Watch that I've "Built". The Blue Dial with Silver White Lume Hands is the Current Version [emoji16]

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Okay Allright, I said I was going to come Back and Share what I managed to Figure Out.

Sorry it took so Long, I've been Busy elsewhere and didn't really have courage to get into Disassembling the Movement.

But I got my hands on another Miyota 8215.

Here's what I did:

I Unscrewed the Date Side Plate on both Movements.

Placed them side by side for Comparison
(One of them was working Properly, the other one had something wrong with either the Keyless Works or Quick-Set Date Mechanism?)

After Tinkering with the broken movement for just about an hour or so, I've decided to try a maybe silly approach and just took out what seemed like a Quick-Set Spring, which in turn solved the whole Issue mentioned above.

I kind of hoped that I'll be able to Share More with You, Sorry for that. This was actually as far as I ever got into a watch Movement [emoji16]

Here are some Pictures of the movement and what I actually took out that seemed to solve the Issue I was Having.

Thank You All for Such Quick and Very Detailed Responses it is always a pleasure to Watch this Forum and Seek Help if Needed [emoji854]

Note: on the Last Picture, I just took Out that small Silver... Spring? Not the Date Ring that remained in the Movement and seems to be working quite OK so far Date has been Changing as expected [emoji4]

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As cheap as these are just but a new movement and install it

 

Thank You [emoji4]

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Hi,

I am facing the same problem as you, I just wanted to you ask something: you removed the Quick-Set Spring, therefore, making the date wheel to not turn anymore, is that correct? I wanted to convert my watch into a no date version, and if the fix is just remove this spring, this should do the trick for me.

Thanks!