I remembered this from reading a post here. The US writes 100,000 but in Europe they write 100.000 for the same amount.
How did this start and has anyone ever tried to make 1 or the other change?
Why do they write 9/11/01 and not 11/9/01?
History part answered here:
R
Be careful about use of the word “Europe”; there is no such singular entity. The UK, for example, does not do this.
why do we answer questions with questions?
Do we?
I thought this was going to be about crossed sevens and tent-shaped ones.
If you look at that article, you will also note that there are more variations in separator and grouping than just two. India, for example, writes one billion as 1,00,00,00,000, and the Swiss use an apostrophe.
Because September 11th, 2001 is a different day than November 9th, 2001? 
For the same reason the muskrat guards his mask. The responsible parties put the ape in apricot, and also make the Hottentot so hot.
Not only the Swiss: Spain and France use apostrophes as the decimal separator, but accept comas because of computer programs which don’t offer the apostrophe among its possible choices.
This is another thing that should be standardized already.
I’m willing to compromise: You Americans adopt the metric system, and we adopt your number style. (Or we could both use the SI style: spaces as separators with dot or comma used interchangeably.)
On the date issue, I’d like to point out that 31.12.1999 makes more sense than 12/31/1999, because the order of the units is not mixed up. I’d like to switch to 1999/12/31, however, since that way you can better sort by date in computer programming. That would leave the abbreviated form in your style.
Padding the zeros is a must, in any case.
Who’s with me?
Most computers can both sort dates properly and display them in the form a user prefers. It’s actually quite useful to distinguish between internal and external data formats, and not require that these be the same.
The tented ones are obviously from their catching one too many surreptitious looks at unclad figure 8’s.
Courage!
Although, as was famously argued in one of the most factually informative ATMB threads on record, ‘Hottentot’ is regarded by those people and their descendants themselves and virtually anyone who cares about courtesy in speech as highly offensive. Its use is somewhat akin to saying, “President Obama’s ancestry is half nigger” – while it is denotatively a factual statement about his ethnic background that in theory could be used non-disparagingly as purely informative, almost everyone hearing it would regard it as an offensive racist slur.
A more modern Bert Lahr would instead sing, “What makes the Khoikhoin act so coy?”
This is the only way that makes sense, and it seems that nobody uses this.
I often put this type of date at the beginning of filenames so I can sort by date.
It’s the only way this makes sense? So all the people of the world have been using dating systems that none of us have ever been able to decipher? How silly of us to futz around not being able to make sense of what we’re trying to tell each other.
I’d say it makes the most sense.
And I was also thinking of filenames and the like, where you don’t have an internal data format. Since the format should be changed anyway 
it’s an added benefit.
The most important thing, of course, is that there is no confusion about what day 2010/05/09 is.
Personally, having spent some years in the UK and also being exposed to European web sites on a fairly regular basis, I frequently get confused about which is the month and which is the date. Sometimes I have to think about it when I write down today’s date. The YYYY/MM/DD is the only system that “makes sense” in that it’s a logical order (consistent with our number system, where we write the most significant digit first), and because it’s unambiguous (because nobody uses YYYY/DD/MM).
Sure they do (somewhat):