#6: A Very Long Year
Okay, in this American’s defense, the differing conventions on writing dates—does it go day month year or month day year?—are pretty arbitrary. There is nothing inherent in either one, I think, that makes it the obviously correct choice, and if you’ve never had reason to come in contact with the alternate way of doing it, it’s not surprising for someone not to be aware of it.

What’s harder to defend is this person’s mind jumping immediately to the idea that other countries have a different number of months. Having interactions with other countries would be really tough if we didn’t all use a standardized calendar (even if some places use their own calendar plus the standard one).