I was looking at the entry for one of my wife's ancestors on somebody else's Ancestry Family Tree. As you can see, the person entered her birth date as 04.11.1817. And her death date as 17.07.1915.
From the death date, it is clear they are using the European system of Date.Month.Year. There is no 17th month. The birth date could either be in April or November.
If you glance down at the Timeline - Ancestry assumes the user was using the American Month/Date/Year format. Oops.
If I were able to copy the whole entry, you'd notice that the Ancestry Timeline gets the death date correct. Which means the software program is coded to recognize the existence of both methods, but only uses the European method if the American method returns an error. I used to be a computer programmer, and I don't think I'd have coded it this way.
Note: I went to my own family tree and tested the interface. There is no clear way to indicate which method you are using, and the software made the same mistake when I entered the same dates. However, the person who created the entry below did make a mistake in entering the date of birth. They cite FindAGrave as a source, which cites her death certificate. Both sources have her date of birth as November 14th. A simple typo, but a reminder to look at the sources.
Further Note: It was mentioned that this programming error may only exist if one uses periods to separate the numbers in the date. So I tested it with the more traditional /s and the same thing happened.