昔のイギリスが月が先に来る形式で、のちにイギリスは欧州式に変えた説が出てきた
> One of the hypotheses is that the United States borrowed the way it was written from the United Kingdom who used it before the 20th century and then later changed it to match Europ
https://iso.mit.edu/americanisms/date-format-in-the-united-states/#:~:text=One%20of%20the%20hypotheses%20is,been%20that%20way%20ever%20since.