GabiMoro: Year, month and day or day, month and year. Everything else has no sense.
Charon121: I agree. The reverse order is especially useful in computing since non-specialised software will automatically sort dates in the correct order if written as 20151007 or 20101231. The existence of different separators (. and /) only adds to the confusion.
Basically YY-MM-DD > MM-DD-YY > DD-MM-YY. The last one is particularly obnoxious because if you remove the separators, then you have this rather annoying amount of work to do in order to reorder it whenever you want to use it.
You have to remove the last 2 digits and append them to the beginning and then repeat that again. With MM-DD-YY, you only have to do that once, assuming that you're not interested in sorting by month.
I'm not even sure what kind of a mind it was that said, we'll order from smallest to largest and never mind that it makes basically everything more annoying, that's what we'll do.
hedwards: The main problem with putting the day before the month is increased cognitive load and it breaking sort algorithms. When I'm confronted with MM/DD for posts, I immediately know whether they happened close to each other or far away. If, I'm confronted with DD/MM, then I have to ignore the first number to look at the second. Then I have to go back to the first number. It's a lot of unnecessary work.
GabiMoro: You are right, MM/DD is more logical, same as hour/minutes. The problem is not MM/DD, but the YY put after MM/DD which make it worse than DD/MM/YY.
Most of the time, the year isn't something we even bother to print. The year goes at the end because it's optional.
If it's not obvious, I wish we'd do it right and put the year at the beginning like the Chinese do. But, placing them from smallest to largest makes little sense other than to satisfy a compulsion. I genuinely can't think of any other reason for ordering them like that.
At least with the SI units, there are things that are improved with their use.
EDIT: Anyways, the reason why it's MM/DD/YY is because when we write the date out long hand, it's October 1st, 2015 and such. Month, day, year. Europeans to be kind of cute decided that they needed to order it by size. Then they chose the wrong one.
:-P