Also: use 4-digit years. There is a forum that I visit a few times a week. It's been around for 20-30 years. It prints dates as 08/11/02, 09/03/04. Very confusing
1. Your browser locale.
2. The developer's preference and ignoring the locale.
3. The server's configuration (so the operators preference).
4. Guessed based on your current network location.
I guess if you are in the US using a US locale it is usually true if you are visiting a US site as almost all of these will select a US locale so it doesn't matter the source, but it is impossible to be sure. And if you live outside of the US or use a non-US locale then at least one of these will frequently return a different locale and you will have little way to know which format is being used.
The month/day confusion is very real, but that isn't what parent complained about. In fact, if the site had used 4 digit years like 08/11/2002, as parent requests, it would still be equally confusing. So that was clearly not the point of contention.