Using Display Format breaks Date Field

The CMS Questions
1

TLDR: Using Display Format in Date Field does not seem to work as advertised (at least for some formats) - instead, the field just stores “random” dates instead of the selected ones

I store a couple of dates with every item on my page. I’m only interested in Year + Month, so this is the basic setup for the field, nothing special:

  project_start:
    label: Starting Date
    type: date
    step: month

Preview will show something along the lines of 2024-10-01 evidently. Well, since I’m only interested in month/year, I’d prefer October 2024 - since that option seemingly exists, I tried using it, by setting display: MMMM YYYY. Whoa!

The value 2024-10-01 was already stored in my content file, so October 2024 showed up as expected. So far so good. Now lets try changing that value: Clicking the little calendar icon reveals the calendar with 2024-10-01 selected - all right. Selecting a different date, lets go for 2024-08-01 - clicking that date in the calendar shows the expected August 2024 in preview, still going strong!

Now just go on to the next field, right? Errr… That’s when the magic happens, sadly. Once the field is exited (which is probably the moment of emission) the value gets jumbled royally. In my case, where a few moments earlier it said August 2024, I got January 2026.

After resetting my content file to 2024-10-01 , I found out another quirk: Even just klicking inside the field (without any modifications to its value), then out would do the same trick: Beam my value to 2026-01-01 - now, if I saved that, klicked back in the field (again, without any modifications) and byck out - next stop: 2026-03-01 which, interestingly seems to be an endpoint (the ouroboros of jumbling dates does not extend further)

Next thing I tried was using display: MM.YYYY - different ouroboros, but same basic jumble: 2024-10-01 → 2025-08-01 …

So… This does not seem to work as advertised. Did I miss something? Is there a config setting involved? I remember reading something about different date handling options (date vs intl vs strftime) anything related to that?
If so: Please, add a hint about that right in the Display Format Documentation, where one might find it… :wink: