There is nothing that annoys me more than the variety of different ways different countries write their dates!
The main problem affecting me occurs with Australian/UK and US formats, there is often no way to tell which is which. Most well designs APIs avoid them both, and sensibly order elements year-month-day, but many people decide dates are simple and reimplement it themselves, fools. The PHP date functions expect things to be ordered in the American way, and blindly produce errors for the unwary programmer.
Java solved the problem by adding layers of confusion. If you want to use dates in Java in a particular format, you need to learn about dates, and then date formaters, and then locales, and then cross your fingers and hope for the best.
Beware the evil dates!